What is SFRM_Materialize_Preferred and how can it be used to write more performant functions?

[Researching this question](https://dba.stackexchange.com/q/301920/2639) , I see there is a value for SetFunctionReturnMode called SFRM_Materialize_Preferred. What is this? Can this be used to write more performant functions? There are multiple occurrances of [SRFM_Materialize in contrib/](https://github.com/search?q=org%3Apostgres+SFRM_Materialize+contrib%2F&type=code) but [none of SRFM_Materalize_Preferred](https://github.com/search?q=org%3Apostgres+SFRM_Materialize_Preferred+contrib%2F&type=code)