I do apologise if I'm missing something obvious but at present I can't discover a way to rename and save a search (for example, by renaming "Search 12" in the tab in which it appears). I'm also guessing, from other comments, that collections are not currently "smart" (in the sense they will not automatically update when new photos matching the collection criteria are added to the catalogue).
My aim is to have a collection of photos matching some search criteria and have that group of photos automatically update when new photos also matching those search criteria are added to the catalogue.
None of this should be taken as criticism. I'm most impressed with PM6+, enjoy using it and I appreciate this is an early beta (I'm currently using beta 2 on macOS).
Stephen