schedule system call

[Ashford Nichols]

In version 2 of the interface under the schedule system call, 
it says that the call can be used be schedulers. However, in the
next sentence it goes on to say that the calling thread must reside
in the same address space as the scheduler. Also, it goes on to say 
that an error code will be set if the calling thread is not the 
scheduler. So is the "schedule" system call made by the scheduler 
or by a thread that simply resides in the same address space as 
the scheduler?