In the docs for TBB, there is a note about reserving slots for 'master' threads in an arena:
https://www.threadingbuildingblocks.org/docs/help/reference/task_scheduler/task_arena_cls.html
A master thread can join any thread 'slot' in an arena, but worker threads cannot join the slot reserved for a master thread. Also, a task arena is allowed more than one master slot. So far so good. What is not clear to me, is what exactly does being a master thread entail? Is it "just" the main application thread? If so, how can there be more than one master thread?
Conceptually, the term "master thread" refers to any application thread that starts a parallel job. In contrast, a "worker thread" is a TBB-created thread which helps master threads process their job.
Specifically for task_arena, master threads submit jobs via the public API of the class, while worker threads join an arena through the internal interface.
A worker thread that joined an arena from inside can also become a master thread in another arena, in case a task it executes in the first arena calls task_arena::execute().