When you're offering something in support of an already existing thing, you're offering something auxiliary. The auxiliary police will help out the regular police with things like directing traffic and crowd control when there's a special event in town.
The Latin word auxilium means "to help," and so auxiliary also means "to help" both in its noun and adjective forms. Anything that helps is fair game. You can have an auxiliary engine in a car, an auxiliary cook at a restaurant, or an auxiliary electrical network at a factory, but it's most commonly used in discussing military affairs with auxiliary troops. There are also multiple groups called the "Ladies Auxiliary" which function as supporting groups, especially in church or for veterans.
