Full Fare is whatever the airline says is Full Fare.

In practice, I'd expect at least:

1. Fully refundable
2. Fully changeable, including routing after departure and name change. (That is to say, fully reticketable)
3. Retains full value even in the event of no-show (for the purpose of changes and refunds)

J2 is full-fare, but I expect a minimum advance purchase of 48h.