White Box [blinking with a number in it]

[Wantedee]

Anyone know what that white box with a blinking number in it is about?

[alienoid]

Anyone know what that white box with a blinking number in it is about?

Bottom left of your screen ? It's a "I" for invitation notification. Someone probably either sent you a friend request, group invite or clan invite.
When it happens and you missed the message, while fighting for example, press your quick menu button, you'll see an "invitation" label in there.