Stability =50 and damping factor =0.9, I think are things, mentioned XDP512 RM
A.5.3.1 XFC Component Selection
The loop bandwidth fC should be chosen to fulfill the Gardner’s stability criteria by at least a factor of 10, typical values are 50. z = 0.9 ensures a good transient response.
I used default 50 and 0.9 for years without any problems. If I had problems with stability or too slow PLL locking, I would study it deeper/