Shanpon (双碰), sometimes written shanpōn, is a tenpai shape in which the hand is complete apart from two pairs, and the winning tile is a third copy of either pair, turning it into a triplet (koutsu) while the other pair becomes the head. Because you already hold two identical tiles of each kind, at most two copies of each remain unseen, giving a maximum ukeire of four.
You can complete a shanpon by ron or by tsumo on either of the two waiting tiles, but the triplet you form is scored differently depending on how it lands. A shanpon win by ron builds an open triplet worth two fu on simples, whereas the same wait completed by tsumo builds a concealed triplet worth four fu, so an identical shape can produce a different final score.
For suuankou, a shanpon preserves the fourth concealed triplet only on tsumo; ron on that shanpon makes the last triplet open and voids the yakuman. Ron remains possible when all four triplets are already complete and the hand waits for its pair on tanki. Watch furiten too: either waited tile type in your kawa blocks ron across the whole shanpon.