==

[Metro28]

I see many modules use Note Off (==), but I never knew what effect it brings, could someone tell me?

[Exhale]

note off == will trigger the note off you have programmed into the sample and the instrument which is often a fade out of some design... (I have used it before for things like echos etc, but mostly it is just a fade out) unlike note cut ^^ which will stop your sample or instrument completely without a fade out.

[Saga Musix]

This is the sort of question that can be answered easily by having a look into the OpenMPT manual before posting on the forum: https://wiki.openmpt.org/Manual:_Patterns#Note_Column