By right clicking the redstone interface connector with a screwdriver access a menu to change what connections the block can access. Unconnected connections show a signal strenght of 0.

Connecting multiple logic units together while still having some channels which are seperate. sometimes 6 sides just aren't enough for connecting individual channels.

What's the usecase?

A logic unit can at most output to 10 different colors. A single connector is enough for that.