You use filename when you want to use the full file name, you use frame when all you are interested in is the frame number
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Note the frame number might be present as metadata in the file even if the filename itself no longer has it. I rename all my files when ingesting, PM at that time stores the frame number in a (PM only) location in the metadata, allowing me to look it up later.
Note thought that as the frame number only goes to 9999, the number is quite useless to me; I easily shoot way more images than that, plus if you shoot with multiple bodies, you'll get mixed numbers anyway.