Attack teammates warning
The Attack teammate? warning message pops up when you choose to attack your teammate in a game. It is a friendly warning explaining that you are targeting a fellow teammate. This dialog box lets you choose between Attack my teammate and Transfer to my teammate.
Whatever selected here will change the "Treat Teammates as Enemies" checkbox that exists on the attack/transfer dialog. This checkbox is ultimately what determines whether or not the attack will attack your teammate - this dialog is just a handy way to check or uncheck the box.