In a multilingual community, people are able to speak a number of languages. This means that they get to choose what language they will use in any given situation. Usually, they come to use different languages in different situations.
For example, one language might become the prestige language. People will use that language in formal situations and when they want to look sophisticated. Another language might become the less prestigious one for use in less formal situations. People's choice of language then becomes a way for them to define various situations, to define whether a situation is formal or informal, for example.
In this way, people can choose what language to use as a way of defining various social situations.