How do clients pick their favourite photos from a gallery?
Turn on proofing. The client opens the gallery, taps a heart on the frames they want, and their selection comes back to you as a list instead of an email describing photos by filename. It is how album design, retouch orders and print selections get agreed without a spreadsheet.
Why filenames do not work
Ask a client to send their picks and you will get "the one where he's jumping, and the two by the goal". Proofing removes the translation step entirely: they select, you receive exactly what they selected.
In SnapFlow
Proofing is a per-album toggle. Selections appear against the album in your dashboard, and a counter updates live as the client works through the set, so you can see when they have finished without asking.