buffer-face-set is an interactive autoloaded compiled Lisp function in `face-remap.el'.

(buffer-face-set &rest SPECS)

Enable `buffer-face-mode', using face specs SPECS.
Each argument in SPECS should be a face, i.e. either a face name
or a property list of face attributes and values. If more than
one face is listed, that specifies an aggregate face, like in a
`face' text property. If SPECS is nil or omitted, disable

This function makes the variable `buffer-face-mode-face' buffer
local, and sets it to FACE.