custom-initialize-set is a compiled Lisp function in `custom.el'.

(custom-initialize-set SYMBOL EXP)

Initialize SYMBOL based on EXP.
If the symbol doesn't have a default binding already,
then set it using its `:set' function (or `set-default' if it has none).
The value is either the value in the symbol's `saved-value' property,
if any, or the value of EXP.