There are plenty of places out there that try to explain how Space-A travel works. This one is the best one that I've come across.
Military Space-A flights were a major element of our strategy for saving money during our year of travel. We flew Space-A to Europe, Hawaii, Japan, and many places within the continental U.S. We estimate that we saved more than $15,000 by taking military flights. Overall, we had great experiences, and whenever it’s a viable option, Space-A is my preferred way to fly.
Talking with others about our travels, we were surprised to realize how few military folks know about flying Space-A, let alone have used it. Even those who knew it was an option were reluctant to give it a shot. They heard you have to wait around too much, the aircraft have too many maintenance issues, or the flights are canceled at the last minute.
There is a whole lot more information at the original page : https://www.poppinsmoke.com/space-a-flights/