Just to add to the above comments and what I understood by 'celldefine: I came across this 'celldefine directive in the libraries used by tmax for running ATPG.
'celldefine defines a module as a cell module, which means, if these modules have any instances of some other sub-modules, then these sub-modules will be masked. For running ATPG, when I read the cell modules, the faults due to sub-modules were ignored.
Everytime I searched for the meaning of 'celldefine, I was expecting an asnwer of this kind.