Definitions for "Production Assistant"
The production assistant generally is a "gofer" that does whatever is needed and is most often given direction by the Second AD. The PA is there to assist in such tasks as bringing actors and extras to the set, giving out vouchers to the extras, keeping track of the extras when they're not working, releasing extras at the end of the day, and whatever else needs to be done. The larger the shoot, the more PA's are needed.
The gopher. Helps the grips, does administrative work for the directory or producer, gets coffee, whatever.
Video crew member who provides general assistance to the production. Roles vary from craft services (food preparation) and transportation, to coiling cables, breaking down sets and running errands for the crew during production.