In the way of trigger systems, there are three options:
AS (Anti-Stress): Traditional Double Action
Basically a trigger system like a Sig, but the P99 is striker fired and hammerless. The AS also has the option of carrying in single action with the trigger forward (the anti stress mode)
QA (Quick Action): Pre-Cocked
Some what resembles the Glock or XD trigger, but slightly heavier with a longer reset.
DAO (Double Action Only)
Name says it all.
In terms of reliablity and function, I have found the P99 to be superior to the XD. I owned a 40SW XD for a time and would take the P99 over it hands down. Sold the XD.