Several workers on one repository at once: the scope a task claims, the board every attached session announces itself on, the worktree each branch lives in, the plan whose status is derived rather than stored, the machines that share one repository, and the models and advisors a worker is routed to. A claim is information and a collision is reported; what refuses a change is the task's own scope, at check and at finish.
Not in this domain: what the work must satisfy, which is governance; and when it counts as finished, which is completion.