Hobson may not be correct. The day you try to use the ticket UA will price a ticket from SAN-LAX and LAX-SMF. If your ticket costs more that the combined 1 ways and you were on a Y/B/M fare, you will be able to apply the remainder of the value of ticket to another ticket. For example I often fly SNA-LAX to ? (for fun lets say LAS). If I get off at LAX on the way home I have an LAX-SNA coupon left. On that route it's usually worth $50 or so towards another ticket because SNA-LAS Y fare is often $400+ but from LAX it's much less.

Clear as mud?