INT 2Fh was the so-called "mux" that various TSRs and drivers could hook into for (in essence) interprocess communication. The half-baked idea was to solve the problem of TSRs commandeering other interrupts for one-off needs, which led to lots of collisions.

In order for the mux to work, each TSR had to have its own identifier code. Other than some reserved ranges, no one organized such a namespace, meaning it was possible for two or more TSRs to intercept the same request, leading to the same collision problem.

This note from Ralf Brown's Interrupt List has the gory details:

http://www.ctyme.com/intr/rb-4251.htm

Incomplete list of TSRs and drivers relying on it:

http://www.ctyme.com/intr/int-2f.htm