I have two doubts in seismic analysis

1. In our past and current projects, in seismic analysis we are using ZPA as 0.385. but if i change that value to any other number (example ZPA=1000) even though the reults are same, then what is the use of that ZPA in seismic analysis

2. In ceasar II config file, MISSING MASS ZPA there are two options extracted and spectrum. but if i select extracted or spectrum, the results are same.

these are the errors in CEASAR II or I did any mistake ????
please clarify my doubts..