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

(custom-initialize-reset SYMBOL EXP)

Initialize SYMBOL based on EXP.
Set the symbol, using its `:set' function (or `set-default' if it has none).
The value is either the symbol's current value
(as obtained using the `:get' function), if any,
or the value in the symbol's `saved-value' property if any,
or (last of all) the value of EXP.