Why is the bottom connector (drain off) of an oil catch tank connected to it ?

I agree it can be connected if the catch tank has a tap and is manually drained back with engine not running.

I want to use an auto return to sump system (with engine running)

This connector will require a pipe with an outlet "lower" than the level of the oil in the sump

Does anyone understand what I am trying to achieve ?

Or am I talking bollox ? - lol lol lol