The default for this option is "extracted", which means that CAESAR II will use the spectrum value at the last "extracted" mode. Changing this value to "SPECTRUM" instructs CAESAR II to use the last spectrum value as the ZPA for the missing mass computations.

You won't see any differences in your static results, this configuration directive affects dynamic runs only. Even then you may not see a difference:
- the spectrum values could just happen to be very close for both settings
- your missing mass component could be very minor, so changing this value has no real effect.

If the missing mass is a significant component of the response, and if the extracted and spectrum settings correspond to different spectrum values, then you would see a difference in the results of displacements, forces, restraints, and stresses.