I understand that a sanitary T cannot be used on its back as a drain fixture. 1. Can it be used on its back as a vent fixture, for example, a 3x3x2 being placed in line with a toilet run to vent? 2. Can it be used to take two horizontal runs (a tub from one direction and a bath sink from another direction) down a single vertical?

Thanks! Andrew

#1 not sure what you mean on its back.
#2. no.

There are two uses for a sanitary tee; one as a drain from horizontal to vertical, as with a side-arm, or on its back, with the tee leg pointed vertical, for a vent.
And the tee should be pointed in the direction of flow. It may not be used in a horizontal position to replace a wye or combination wye and eighth bend in drainage. Regards, Tom