Assignees

An assignee is the person responsible for an issue. See /:owner/:repo/assignees.

Assigning

An issue may have more than one assignee. Assigning does not notify anyone today; there is no notification system yet.

Unassigning

Removing an assignee leaves no trace on the issue. If you need the history, say so in a comment.

A current limitation

The assignable-users endpoint returns an empty list, so no user is reported as assignable even though assignment itself accepts any login. This is a known gap rather than a permission rule.