The IVOA Astronomical Data Query Language (ADQL) has the extension facility of user defined functions (UDFs). While service operators are free to define these as they see fit, it is clearly advantageous if identical functionality is available under identical names in different services. Therefore, ADQL 2.1 has introduced the ivo_ prefix, reserved for functions with agreed-upon semantics. This endorsed note contains this agreement and, via its updates, is the means of updating it. A non-normative appendix lists UDFs declared by registered TAP services; some of these might be developed into consensus UDFs in later releases.