The individual bolt assembly elements , such as bolts of different sizes and lengths, nuts and washers, are listed in the bolt catalog. Each bolt assembly then consists of these bolt assembly elements. You cannot use a bolt if it does not belong to a bolt assembly. The bolt assemblies are listed in the bolt assembly catalog.
Tekla Structures stores the bolt catalog information in the screwdb.db file and the bolt assembly catalog information in the assdb.db file.