24 is not automatic. IF they are offering it, then you'll get some detailed documents explaining all the ramifications. The choice is up to you to accept or decline the 24-year offer.

Continuation rules are set for each board....check the AFPC website about the announcements for this upcoming O-5 board, and there may be mention of what AFSCs will be offered the extra continuation. If you can't find it there, then at the very least you'll be told what your continuation options are when the results are out and you receive your "sorry we didn't promote you again" letter. If your AFSC is not being offered 24, then you'll probably just get the passover letter and nothing else. If your AFSC is eligible for continuation to 24, then you'll get the detailed documents mentioned above.

When I was offered continuation to 24, I had the option of declining that and keeping my existing 20-yr DOS - which is a mandatory separation (retirement) date.