[SAM] Investigate billing behaviour for custom minimal access role

Background

SAM introduces explicit seat assignment to manage seat usage, where:

• Members/Users only consume a seat when explicitly assigned one
• Seat assignments will have a "seat type" that determines available roles for the user (e.g. maintainer, developer, etc) when they're added to a group or project
• Simplified billing calculations and improved customer control via the newly added seat assignments

In https://gitlab.com/gitlab-org/gitlab/-/issues/550788+ we've identified a need to clarify how members with custom roles based on minimal access are billed, both on GitLab.com and Self-Managed instances.

Proposal

1. Determine the current billing behaviour for members with custom roles based on minimal access:

• On GitLab.com
• On Self-Managed instances

Additionally, we should check this in CustomersDot as well. Since this is where the customer is eventually billed, one might say it is the SSOT.

Next

The findings will inform the implementation of seat type identification for custom roles in https://gitlab.com/gitlab-org/gitlab/-/issues/550788+.