Credit usage covers access to compute. We prefer to keep usage costs as low as possible to encourage frequent job runs, which is the foundation of a good CI practice. Per-active-user fees cover access to platform features and job orchestration. This includes features like dependency caching, artifact caching, and workspaces, all of which speed up build times without incurring additional compute cost. Our per-active-user charge also allows us to provide support to all customers and deliver additional features like insights and orbs.
Anyone who triggers a build on CircleCI is an active user, regardless of whether they have a CircleCI account. If a user without a CircleCI triggers a build, for example via a pull request on a repo, they are counted as an active user.