Pan is modal: it stays active until you right-click or select another command.
To constrain panning of any viewport to a single axis, hold down the key. The pan is constrained to the axis you first move while the key is down.
To accelerate panning, hold down the key.
You can also pan by dragging in a viewport while pressing the middle button of a three-button mouse. This lets you pan without turning on the Pan button.
To pan a viewport:
To pan a non-Camera viewport:
To accelerate panning:
To constrain panning to a single axis:
The pan is constrained to the first axis you use.
If you drag vertically at first, the pan or orbit is constrained to be vertical; if you drag horizontally at first, the constraint is horizontal.