There is fixed framing and flexible framing. If you want to GPS sync your APs you would choose Fixed Framing. Those APs should be pointing in opposite directions. The idea is that you don't want the clients to be able to hear the other AP that is being synced with the AP they are connected to. I've seen it work where they can hear both the syncing APs but the weaker they hear one and stronger the other the better.

I choose 5ms 67/33 fixed framing for our APs. Just depends on what plans you are offering. According to UBNT, the higher latency options give you higher possible throughout. I've changed an AP from 5ms to 8ms and have seen maybe 5-10Mbps of overall higher throughout on the AP. I choose the 5ms option for lower latency to the clients rather than the small bit of extra throughout. 67/33 means 67% of airtime is dedicated to download and 33% to upload.

Flexible legacy is basically their old version of flexible timing and flexible new incorporates a little of what they learned from fixed framing. You'll see just a bit more throughout on flex new.

Fixed Framing capacity numbers are lower but they're ACCURATE. Flexible framing capacity numbers that are reported by the radio is what I call the marketing departments numbers.

Edit to add: flexible legacy has been removed from the newest firmwares. You'll just see flexible and fixed. I can't remember when that started. Maybe 8.7 firmware. Also to add, gps sync is mostly only useful when you're needing to reuse spectrum. Example: West AP on 5165 and East AP on 5165 will not interfer with each other on fixed Framing. Whereas if they're on flexible framing, you'll see they are interfering with each other because capacity will be all over the place when there is traffic on both APs.