If you do not define any region-specific sensitivity parameters in your sensinput.dat control file, you will not need to define a CMAQ_MASKS file in your CCTM run script and you then also should not uncomment any lines referring to CMAQ_MASKS in the &Desid_RegionDef section of CMAQ_Control_DESID.nml
If you do want to specify sensitivity parameters for specific regions, you will need to set these variables and point them to a corresponding file defining these regions. Region files used in DESID emissions scaling as well as DDM-3D and ISAM are described in Appendix B.
You can also search this forum for previous threads discussing approaches to create such files by using a search term of “mask” or “region”, e.g. here or here