Why RACES stands for the Radio Amateur Civil Emergency Service, a formal part of the amateur service defined in FCC Part 97 (see 97.407). Licensed amateurs enroll with a local, state or federal civil defense or emergency management organization, and RACES stations provide communications for that agency during emergencies, including a presidentially declared national emergency or wartime when normal amateur operation could be restricted. It is government-sponsored and rule-based, unlike ARES, which is an ARRL volunteer field organization.
Watch out RACES does not involve citizens band radio or CB frequencies at all; it operates only on authorized amateur frequencies by licensed amateurs. The contest and experimentation society choices simply play off the word 'races' and the letters.
RACES = Radio Amateur Civil Emergency Service: civil defense, in the FCC rules. ARES = ARRL volunteers.