Okay this version of the rules is clearer on the matter of Reserves than the last with one major exception.
When a character leaves a military career he is in the Reserves.
What is not explicitly stated anywhere that I can see is that the character remains with the reserves until Life Stage 9 (inferred from the table on p.48) when they retire from the reserves and start receiving a Reserve Pension.
It's important to understand that a character remains in the Reserves for this period because at anytime "characters in the Reserves (from participation in OTC/NOTC, or Service Academy) who roll an unmodified 2 exactly do not continue in the current career, but return to their military career for the next term, at their previous rank."
Somewhere in the text it should say "a character who leaves a military career is automatically in the reserves until retirement at Life Stage 9 when he/she receives a Reserve Pension".
Which brings up another question. Can you resign from (or be cashiered from) the Reserves?
If so this would explain how "Cashing out" on p.51 applies to a Reserve pension. It would also avoid the possibility of being reactivated in a military career if they player wants to avoid that possibility.
(Probably a lot of the above applies to other pensions and careers but I'm specifically interested in the Reserves).