An "Active Contributor" in GitClear is any user who has made commits to your git repository in the past 30 days. Normally, customers want to process stats for all of their Active Contributors, but GitClear offers several options to opt-out of being billed for developers who have made a recent commit.
On this page, we will describe "how does GitClear choose when to assign a seat to a developer?", "how can you control which developers get seats?" and "what happens when a developer doesn't have a seat?"
Here are all these topics, in case you know which specific section will answer your question:
For a contributor (developer) to have their stats processed, they must possess a Subscription Seat.
During the trial phase of a subscription, a Subscription Seat is automatically granted to any developer who have authored (why "authored" vs "committed"?) a commit in the past month. If the manager doesn't want to see activity for these committers, they can change which ones receive seats in their Contributors Settings.
When a company finishes their trial and becomes a standard paying customer, they will initially purchase a seat for each recently-active developer among the repos being processed, except for those who had their seats removed. The current count of subscription seats is shown prominently during the trial sign up process.

During signup, you'll be able to confirm how many contributors are going to have their commits processed & stats available
Upon completing the checkout process, a user can return to Settings => Billing to manually adjust how their count of subscription seats will change.
By default, subscription seats will automatically increase or decrease based on how many developers have recently authored work in the repos that you process. Any time you visit Settings => Billing, you will see how many seats your next renewal would include:

A team that allows "Automatically Adjusted Seat Count" will have their cost decrease if their team size, or count of active repos, does
Some customers want to ensure consistent costs, even if they import more repos or add more developers to their team. To accommodate this use case, the user can click "Manage subscription" from "Settings" => "Billing" and unselect "Automatically adjust seat count"

After becoming a subscriber, you can ensure a fixed number of developer seats are renewed
If "Automatically adjust" is not checked, new developers will not be processed unless another developer stops contributing. If a developer leaves the team, the seat count will not reduce if the "Automatically adjust seat count" is not enabled.
By default, any contributor that has been exiled, or who has previously had their Subscription Seat removed by an admin, will not receive a Subscription Seat. That said, it's not uncommon for teams to only wish to process git stats for a subset of their active developers. This can be controlled by visiting Settings -> Contributors and clicking either the "Active Seat" logo, or the dropdown option from the right side of the row:

Upon clicking "Active Seat" or "Release Seat," a menu will open allowing you to reassign the Subscription Seat:

If you choose a specific developer, they will receive the Subscription Seat and will immediately have their stats begin processing. If no specific developer is chosen, then GitClear will automatically assign the open seat to the Active Contributor with the greatest amount of commits over the past month. A contributor who has their seat relinquished by an admin will not have a Subscription Seat automatically assigned to them, they will only begin processing stats again if the admin returns to the Contributors list and chooses "Acquire seat":

Choosing "Acquire seat" will utilize an existing Subscription Seat, if you have a one that is not utilized by an Active Contributor. If all of your current seats are allocated, then we will increase the count of seats to be renewed, such that this developer's stats will begin to show up on GitClear charts.
On behalf of saving space & memory in GitClear database -- as little as possible.
When a developer doesn't have a seat, their work will not show in Commit Activity Browser (commits are designated "ignored"), Pull Request Stats, notifications, or Delivery Velocity graphs.
The "shell" of the commit (its sha, and its parent shas) will still be generated, such that the coherence of the git tree is maintained. However, no lines or files are processed in commits made by an unseated committer. Since their commit lines are not processed, it is not possible to render a Commit Group when one or more of the commits in the group come from an unseated committer.
Since there are a lot of interesting and complex scenarios that can arise with real world teams, these examples help to illustrate how Subscription Seats are calculated under maximally complex circumstances.
During your trial, GitClear finds 3 contributors who have authored code in the last 30 days across all of your repos, and 10 contributors who made earlier contributions. The contributors who made earlier commits will factor in to some stats (like File Directory browser), but will not show up in Delivery Velocity Stats, Issue Stats, or Pull Request Stats. You purchase an annual subscription at the "Pro" level after your trial, which gives you three subscription seats for 12 months. After 2 months, one committer departs. Their stats will continue to be processed since you have 3 seats. Then the departed developer is replaced, and a new developer becomes active in your repos. At this point, GitClear will automatically remove the Subscription Seat from the departed developer (whose stats will no longer be present) and will begin processing stats for the new contributor. In this case, since there were never more than three Active Contributors at once, the "Automatically increase seats" option did not come into effect.
When this company began their GitClear subscription, they purchased 400 annual licenses, with "Automatically increase seats" disabled. The company then hired 5 more developers, but did not want to incur the additional pro-rated cost of buying 5 more GitClear seats (if they did purchase 5 more seats, they would only be charged for the portion of the term between "current date" and "next subscription renewal"). To ensure that these new developers' stats were shown within GitClear, the CTO decided to relinquish his own Subscription Seat (since he rarely makes commits) and the Subscription Seats for 4 other Managers who only made commits a few times per month. These intermittent contributors will no longer show up in places where stats are processed, but the 5 new contributors will now have their data processed.
The relinquished seats will automatically be assigned to the new contributors within the following day, but it's also possible to begin processing the new developers' stats immediately upon relinquishing the Subscription Seat from the Contributors settings page:

Choosing which Contributor will inherit the Subscription Seat
In this example, the company paid for 10 Subscription Seats at the Pro level during their previous renewal, 6 months ago. The new CEO has decided that they want to unlock the extra powers that come with an Elite subscription, but he wants to begin paying on a monthly cycle due to spread out costs.
The first option is to wait until the 6 months are finished, and let the Elite subscription kick in at that point. But if the company wishes to upgrade to Elite now, they can do that by paying the pro-rated cost of upgrading the existing seats until the end of the current renew interval. The first step is to visit the "Manage Subscription" page, choose "Elite" as the selected level, and choose "Monthly" as the billing interval. This will produce a link to "Review Changes" where the admin can decide whether they want to upgrade their subscription now or later:

Configuring a subscription to upgrade to Elite while downgrading to monthly billing
The following page describes this upgrade situation:

This price is calculated by taking the difference per month between Elite and Pro ($10), multiplied by 10 contributors over the 6 months that remain in the current billing cycle: 10 * 10 * 6 = $600 to upgrade all the contributors to Elite through the end of the current term. The subscription will begin to be billed monthly, at $260, thereafter.