A smart folder looks like an ordinary folder in the sidebar, but nothing inside it was dragged there. It's a saved search — a rule like "everything tagged portrait added in the last month" — and the app re-runs that rule every time you open it, so the contents change on their own as files are added, retagged or deleted elsewhere. macOS Finder has had them for years; several photo and asset tools ship their own version under the same name.
What makes it different from a real folder
A file can sit in exactly one real folder at a time — that's what a filesystem folder is. A smart folder doesn't hold anything; it just asks a question and shows whatever currently answers yes. The same image can appear inside several smart folders at once — one for its tag, one for its rating, one for the month it was added — without existing in more than one place on disk. Move the real file to a different folder entirely and a smart folder built on tag or rating never notices, because it was never looking at location in the first place.
What Coii Ref calls the same idea
Coii Ref doesn't have a feature labelled "smart folder," but its saved views do the same underlying job: store the text of a search rather than a snapshot of its results, and re-parse it the moment you open the view. Rename a tag across your library and every saved view built on that tag follows the rename automatically, the same way a smart folder would pick up the change without anyone editing the rule by hand.
Eagle is the more direct comparison here — it ships smart folders that keep themselves current by name, tag, colour or format, covered in the full comparison against Coii Ref, where a saved view built on the query line is the nearest equivalent on our side rather than a like-for-like match.
When a smart folder actually earns its keep
A rule-based folder pays off once a library is too large to refile by hand every time something changes state — a shoot gets a final rating, a project wraps and its images get archived, a client approves a batch. Rebuilding that list manually every time is the tedious part a smart folder or a saved view exists to remove; for a library small enough to just scroll, the extra setup rarely earns its keep.
The same idea, a different name
The choice between organising with folders and relying on saved rules like this one is really the tags-versus-folders question wearing a different name — see what a reference manager is built to do for how the two ideas fit into the wider category.
What breaks a rule-based folder
The failure mode worth knowing about is a rule built on something that changes for reasons unrelated to the library — a folder path, say, rather than a tag or a rating. Move the underlying files around outside the app and a location-based rule quietly stops matching what it used to, with no error to flag it. A rule built on tags, ratings or dates survives exactly that kind of reorganisation, because none of those properties live in the filesystem path at all.