The exclamation mark at the beginning of a pin's name on a schematic symbol indicates that the pin has an active-low logic. This means that the pin will have an active state when the logic level is 0, and inactive when the logic level is 1.

The exclamation mark serves as a visual cue to help users notice this in the symbol. In some CAD design software, the active-low logic can also be defined with an overlined pin name.