After cutoff frequency , if we select the option Include Missing mass then only the quasi static response of non extracted modes to be combined with modal response to get the total system response .The ZPA used for missing mass is not the ZPA as entered in the control parameters.This is automatically selected from the spectrum based on the option selected by you in configuration file.This ZPA is used to calculate the missing mass residual load (load related to non extracted modes). This residual load can then be applied statically to get the resulting displacement,stresses and reactions representing the net response of all rigid modes.This response is then added to the response of the extracted low order modes to get the total system response.

You may also say that extracted modes response are dynamic(or amplified response) and non extracted modes response are approx rigid (non amplified) range. The response of non extracted modes are rigid based on the rigid ZPA value as selcted by Caesar II for missing load calculation.
I suggest you to read the description regarding include missing mass as provided in Caesar II help.