Our arena is both in/out. 1/2 is under roof/enclosed. Hard to describe.
Anyway, the footing obviously has to be the same for both parts. It is 1/3 concrete (C-33) sand, 1/3 screenings (called blue stone or crushed limestone in other areas) and 1/3 Permaflex shredded rubber.
I LOVE it.
Oh, and a 9" compacted screenings base that drains to one outside corner. With french drains installed around the outdoor perimeter.