The ‘exact’ and ‘substructure’ settings for the Structure Search application are intended to allow easy searching for either complete molecules or fragments of molecules respectively.
Exact match will only find hits for complete molecules. This will automatically add any additional hydrogens as a constraint necessary to complete neutral, organic molecules, as well as constraining the number of bonded connections per atom. Bond types are also used as search constraints where specified.
Substructure match will find hits containing the drawn substructure. This will find matches of the drawn query as a substructure within any larger chemical unit as long as the query constraints are met. Bond types are also used as search constraints where specified.
For example, drawing a simple phenyl ring in the sketcher using the ‘Add Phenyl Ring’ button and searching using the ‘exact’ setting will only match structures containing the complete molecule of benzene, for example a benzene solvate structure. The same search using the ‘substructure’ setting will find structures containing any kind of substituted or unsubstituted phenyl ring.