Concealed tiles have not been shown to the table. In American play they stand on the rack; exposed groups sit in front.
After you call a pung or kong, that group is exposed. If the card wants a concealed group, calling it breaks the line.
Chinese tables also distinguish concealed kongs and a closed hand. Riichi is declared only while the hand is still closed and one tile from finished.
American mahjong has no riichi declaration. It reads a printed line, and whether that group was exposed.
See call.
Once you call and expose it, the group is no longer concealed.