Google AI told me this today:
A "set.php?id=" link on Numista.com typically refers to a specific coin set within their catalogue. The "id" parameter likely represents a unique identifier for that particular set in the database.
For example, the link https://en.numista.com/catalogue/set.php?id=327 refers to a specific unpublished set on Numista. Similarly, https://en.numista.com/catalogue/set.php?id=531 leads to another set, this one described as being composed of three silver coins.
While you cannot currently search specifically for sets within the Numista catalogue, this functionality is reportedly planned for future implementation. To find information on sets, you'll generally need to browse through individual coins and check if they belong to a set.
Mr. Google is watching.