On Aug 22, 2007, at 11:32 AM, Mcnutt, Larry E. wrote:

TSM Server 5.3.3.0
AIX 5.3

This is probably not a TSM specific question, but hopefully someone
can
point me to the right place to look.
We would like to understand how the library decides which available
drive to use. ...

It doesn't decide: the host determines which drive to use.
TSM uses them rotationally, to balance wear.

For a 3494 library, see the MTIOCLM (Library Mount) library system
call in the IBM SCSI Device Drivers Programming Reference for the
nitty-gritty.  For a SCSI library, where the host has to engage in
more intricate control of library mechanics, I believe it's the
SMCIOC_MOVE_MEDIUM call (Transport a cartridge from one element to
another element).

   Richard Sims