Andy Brownsword stakes a claim:
It’s a function, but not quite as we know it. They’re surrounded by { curly braces } and prefixed with fn. The kind of thing I’d expect to see in PowerShell.
It turns out they were introduced in ODBC versions 1-3, with the latter being based on the SQL-92 specification. These were introduced in 1995 and 1992 respectively, which likely explains why I don’t see them used.
I’m thinking hard and don’t believe I’ve ever seen anyone use these in code I’ve maintained. Thus, I agree with Andy: you probably don’t need them.