p_top_requested for non-isobaric dataset

For pressure level data p_top is the top level or lower but what happen with non-isobaric data.
What is the value I should set for p_top_requested in namelist.input in this case?

P_TOP is specified for WRF vertical coordinate. Your input data has nothing to do with P_TOP except that p_top must be lower than the highest level of your input data.
By the way, If your input data is not on isobaric levels, I believe that you still need to provide pressure levels corresponding to the levels where the data are located.