Attack teammates warning

The Attack teammate?-warning message pops up when you choose to attack your teammate in a game. It is merely 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" check-box 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.