Attack teammates warning
The Attack teammates 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 Attacking your teammate or Transferring your troops to them.
There is also an additional checkbox below to never show this dialog box again. If you have checked the box in the past and you want to have this dialog show up again you can make that setting change in the Settings tab.
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.