A bit like the ambiguity of search facets: if I select one facet, I get results that match, but if I add a second facet, should the results expand (OR'ing my selections) or contract (AND'ing my selections)? Presumably they should be OR'd if they belong to the same category (like selecting multiple colors, if any given result has only one color) but AND'd otherwise (like selecting a color and a size). But then a category could consist of miscellaneous features, and I want results that have every feature I've selected, which goes against the general case.