I have originally posted this on the wrong Lightroom forum, so I apologise if you use both forums and see this again.
Originally, it was all about restricting a smart collection filter to a particular LR top folder. It then spilled on more general observations on LR filtering in smart collections.
Say I have 2 top LR folders D:\pictures and D:\videos.
I want the query to get the items from D:\pictures only.
Folder Contains "pictures" will return them all but will also include any LR folder with "pictures" in it, e.g. a video in the D:\videos folder called "videos showing my pictures of the holidays". That is normal, expected.
So I changed the smart collection to Starts with "pictures" and Ends with "pictures". That is unique but it still returns the same. I made sure to use the "All conditions" options which I assume is equivalent to an AND.
I have to play with starts and finishes because the option "is" or "equal to" is not there. The same apply to keywords for example.
The options "contains", "contains all" and "contains words" all 3 return the same inflated answer. That is "sort of" expected from them 3, but they are ambiguous or superfluous, depending on your interpretation. In my mind, they are the same and return the same as I am expecting. I would happily scrap the last two and get one "is" or "equal to" in exchange.
I resorted to assign keywords like "Dpicturesfolder" for the whole D:\pictures but that is not "smart" as I must make sure keywords are always up to date.
I could change all occurrences of "pictures" and refrain using it in the future but that is not smart either: I have many of those top folders with commonly used words like Documents, Music, Events, Travel etc. for example and I cannot go round telling people not to send me pictures sets with these words in folders.
I could make a collection of the top LR folder with a unique name but that has the same drawback at the keyword solution (it needs to be kept up to date).
I could also resort to rename D:\pictures to "D:\top folder pictures" or something else unique but where will it end to sort out what seems like a bug or a misunderstanding/ lack of knowledge of mine.
So, plenty of workarounds, but they are all dodgy when it should be straight forward. I might be missing something here but, in that case, I still need help.
Thank you for your comprehensive answer to my rather long post.
You are surprised that an "is" option has not been provided by LR yet and one your suggestion for my problem is a plugin (Any Filter). It is not the first time that I have to go to a plugin to get a functionality which should be there in the first place - Listview being another example. I hope that product managers do not rely on someone developing a plugin for essential basic "for everybody" stuff. That would of course not be managing a product in my view.