Double threaded vs single threaded bulkheads?

Question

I am confused whether I should get a double threaded or single threaded bulkhead to attach to the built in overflows and stuff on my fish tank.

When should each be applied?

Answer 1

A double threaded bulkhead is used when, you need to attach two fittings to it. One on the inside, one in the outside.

A single threaded bulkhead is used when, only one pipe is needed.