This label can be used in a variety of contexts. Typically used in table elements when it has sorting capabilities.
One example use case is the Users table, where you can see a list of users on Discourse. Clicking on the labels at the top will allow you to sort by a particular criteria. Though the exact string is not seen on the user interface, it is used for screen readers.
For example, when sorting by Username the string is used like so:
Sort by %{criteria} → Sort by Username
and the text is announced to screen readers when toggled.
Still, this is just one use case, and the string may be used in other tables/other contexts.