For IC components with exposed pads, the number of the exposed pad should start one greater than the pin-count of the footprint.
For a SOIC-8 package with a single exposed pad, the exposed pad will be assigned the number
Shielded components use "SH" as the pin number for connecting the shield. For a pin to qualify as the shield pin it must be connected to a metal enclosure. For connectors it must connect to the shield of the cable.
Parts that include mechanical mounting pads without electrical use, get the pin number "MP". Non plated through holes do not get any pad number assigned. (They do not get a pin in the symbol.) An example for such a pure mechanical mounting pad would be the pads for the two metal leads on the side of Molex picoblade smd connectors.