Busses as output path destinations?

[AE]

from the way it reads in the pro tools reference manual, the destination of an output path defined in the i/o setup dialog can be either an audio interface output channel or a bus. here's a chart copied from page 49 of the 7.3 guide:

Path options by type
Input: Names, formats, and source channel (analog or digital audio interface)
Output: Names, formats, and destination (audio interface output channel or internal send bus)
Insert: Names, formats and destination (audio interface channels)
Bus: Names and formats

it would be nice to be able to route output paths to busses... but how is this done? i can't find any support for this within i/o setup. i only have access to an mbox at this moment, but can't recall this capability with hd either.

what am i missing?

(note: i'm not asking about track output assignment to busses. but this would allow you to, say, change your master mix output that you have tracks assigned to, and simply re-route these to a stereo bus within i/o setup. that would be useful...)