OpenBSD threads priorities

Can anybody explain how to work with thread priorities in OpenBSD? Information about this question in MAN pages is poor, it does not give clear understanding. In particular: sched_get_priority_min/max gives range. I have OpenBSD 6.3 amd64, and range is 0-31. If i see default priority policy/level of first created thread of process, i see: (SCHED_OTHER, 0). If 0 is lowest, how can i specify idle thread? Or priority below normal? Can i only **increase** priority? Which priority have system threads? By idea, default priority level has to be normal. MAN page pthread_setschedparam() says: "The scheduling policy for a thread can either be SCHED_FIFO (first in, first out) or SCHED_RR (round-robin)". Why not SCHED_OTHER? First default thread has **this** policy. Are these priorities for real-time or are they normal? Can anybody make it clear? In Windows i can set idle priority, and i see that this thread is scheduled only when there is no other work in a system. I can set time_critical, and this thread has almost all processor time. How to resolve these tasks in OpenBSD? What are analogues of Windows priorities: idle, lowest, below_normal, normal, above_normal, highest, time_critical? How to make choice between policies? FIFO, OTHER or RR? Are these policies equal in granting processing time when priority level is the same? A lot of questions without answers in MAN pages.