Opplayer messages are fired whenever a player clicks an option on another player, or if messages such as "* wishes to trade with you." are clicked. In the case of latter, only ops 1, 4, 6 and 7 will fire the packet.
OpPlayerT messages are fired whenever an interface component is targeted on another player, which, as of revision 204, includes using items from the player's inventory on players - the OpPlayerU message was deprecated.