Wannabe,

If a squadron CO (Commanding Officer) says that a certain pilot will never sign for an aircraft in his squadron, he means that the pilot in question will never act as the aircraft commander (Captain/PIC, in civilian terminology). It's a pretty harsh thing to say about someone, and it speaks loud and clear as to what the command thinks about a pilot's leadership and flight skills. It basically means that the command doesn't trust the guy.

Clear as mud?