Crew Management - efficience question

Nothing about this item in the manual so it's probably one of those things conspicious by it's abstenence, etc.

Just got back from Mission 10, some damage to the hull, etc, and a few injuries during a depth charge attack by some clown that found me (even though I was sitting at PD with nothing running and silent running).

So I'm in the office going over the crew listing and notice that the efficency listing on a number of the crew is less (?) than the new people that I could possibly get I wanted to replace them.

What exactly IS that rating supposed to mean? How am I supposed to interpret the numbers and that 'k' thing after it?

Do they (the crew) get 'old' after a while and need to be dumped? There's a buncha guys that have 100's in guns, which is why I like to keep them (that deck gunner is a real butt-kicker, and those AA guys can knock a seagull out of the sky at 200 yards).

But seriously, do they need to be replaced like a worn-out cog in a well-oiled machine?

Damn, I hate paperwork!!