Space Available is just that, if there is space available on a flight, eligible personal can catch a ride. There is a priority list of who gets those spots.
Active duty with orders top the list, then active duty on leave, then retirees. Obviously any type of VIPs would likely be above active duty on leave.
I don't remember getting any kind of briefing when we flew space A. Though we were just on cargo planes, never the smaller passenger planes with seats like a commercial plane. That could make a difference.
After taking a C-17 to Germany, and spreading a blanket and pillows out on the floor for a nap, I never, ever, want to fly commercial again.
The C-130 crew let my two youngest come up front for take off, and the load master gallantly handed me an airsick bag when I started to turn that special shade of green. As big as those planes are, they rattle and shake quite a bit.
I remember seeing parachutes, but no one instructed anyone on their use.
Because the C-17 was carrying several families with small children (on their way to Ramstein to see injured active duty family members, as I recall), the pilot turned the heat on. Normally you would freeze your butt off.