Questions about traveling Space-A, Mac flights, or whatever?

I am stationed in Rota, Spain, with my husband who is active duty. I have a 9 month old daughter, and want to go home to Washington. I have never traveled Space-A, and I am very nervous traveling with my daughter on a military flight. Can anyone offer any advice, or tips for me? How it works? Thanks!
Going home to Washington State :)

I'm presuming A means available, if this is right, it means what it says, if a seat is available you will get it, if not, wait for the next available seat.
I don't know how the military work it, but for civilian airline staff you turn up at the airport and take the chance, often meaning a last minute run to the departure gate or holidays being cut short or extended.