custom-command-apply is a compiled Lisp function in `cus-edit.el'.

(custom-command-apply FUN QUERY &optional STRONG-QUERY)

Call function FUN on all widgets in `custom-options'.
If there is more than one widget, ask user for confirmation using
the query string QUERY, using `y-or-n-p' if STRONG-QUERY is nil,
and `yes-or-no-p' otherwise. Return non-nil if the functionality
has been executed, nil otherwise.