While performing a c-find for studies (I only have this issue with CT studies) based on
ModalitiesInStudy='CT' Impax returns 41 studies for the past week
Modality = 'CT' Impax returns 464 studies for the past week
ModalitiesInStudy='CT' AND Modality='' (empty string) Impax returns 538 studies for the past week.
Based on the information from the PACS client the last one seems to be correct.
Now the interesting part is although it seems to match on ModalitiesInStudy correctly Impax returns an empty value for this tag using the last request.