macnala - 2012-08-02

What is the meaning of the icons which appear adjacent to the differences between two files?
The + (in circle) is obvious as is the - (in circle) I can assume thet the ! (in triangle) means some sort of warming
but exactly what is being warned of.
I have tried to see the difference between the two files I have but cannot see why there is the ! warning icon.
No tip appears when mouse hovered over the icon - which would be a good improvement.