Axis doctrine altitudes?

[MarkShot]

There is 9,000, 20,000, and 22,000 feet. I understand for tactical bombing that flights will descend, and escorts should remain at higher altitudes.

I remember SDG explaining upon release that 9,000 was the best altitude to avoid neighbor ingress/egress HEX flak.

Would anyone care to elaborate on the default choices of 9,000, 20,000, and 22,000?

Thanks.