Follows same definition as cum_on_clothing:

"This tag is used when there is cum visible on the clothing of a character."

Could apply to the following:
(1) a depiction of a character wearing underwear with cum on its outward facing side, and/or

(2) a depiction of a character wearing significantly lowered, partially removed, or fully removed underwear with cum esp. from an external origin on any part of it (e.g. lowered underwear with cum dripping down onto it, or cum from another character), or

(3) cumming on unworn underwear

For cum on the inside of partly pushed down underwear, refer to cum_in_underwear