The download creates a specific layer structure. Usually, this results in a folder or a group of layers (Low Frequency and High Frequency) that sits on top of your background. This keeps your retouching non-destructive, allowing you to hide or delete the group to return to the original image.
While the original developer site may sometimes be unavailable, reputable educational resources like Noble Desktop provide access to the specific .atn files as part of their professional retouching curricula. Fx Ray Frequency Separation Download