Wye on it's back?

I have seen it referenced many times that a wye fitting cannot be on it's back.
Just exactly what does this mean?

A tee fitting can't be on it's back.

A wye fitting can.

Could a wye be used instead of a tee - in general?

Yes, unless you are venting on the vertical.

Waste: vertical to horizontal, wye, or long turn
Waste: horizontal to vertical, wye or tee
Waste: vertical to vertical, wye

Trap arm venting: horizontal to vertical, tee
Trap arm venting: horizontal to horizontal, wye
Vents on the horizontal use wyes, vents on the vertical use tees.