An example of this would be replying to a read() where all the data existed in a single buffer. You'll typically see this done in two ways:
return (_RESMGR_PTR(ctp, buffer, nbytes));
And:
SETIOV (ctp->iov, buffer, nbytes); return (_RESMGR_NPARTS(1));
The first method, using the _RESMGR_PTR() macro, is just a convenience for the second method where a single IOV is returned.