reftex-arg-label

reftex-arg-label is an autoloaded Lisp function in `reftex-auc.el'.

(reftex-arg-label OPTIONAL &optional PROMPT DEFINITION)

Use `reftex-label', `reftex-reference' or AUCTeX's code to insert label arg.
What is being used depends upon `reftex-plug-into-AUCTeX'.