The namespace presets can be a convenient shortcut to easily filter search results on a specific scope. But for users it's not very transparent what they do what they do exactly unless you click them. Even then it's just communicated visually what's happening and which namespaces will be added automatically to the selection list.
It might be useful to have a general help for that section of settings, or at least a descriptive title attribute for each preset that makes more clear what's happening and maybe even which namespaces are added exactly.