Lexical filtering is a feature which allows the user to constrain the lexical selection to only those items that contain a certain property, for example, the realising an item as a cleft.
The idea is that the user provides an input like idxconstraints:[cleft:j], which means that the lexical selection must include exactly one tree with the property cleft:j in its interface. This mechanism works as pre-processing step after lexical selection and before polarity automaton construction, in conjuction with the ExtraPolarities mechanism. What we do is
Preprocess the lexically selected trees; any tree which has a a desired property (e.g. cleft:j) in its interface is assigned a positive polarity for that property (+cleft:j)
Add all the index constraints as negative extra polarities (-cleft:j)
Note: To use lexical filtering, polarity filtering must be enabled.