After evaluating a selection expression, the result is compared against one or more selection keys to find a matching key. See <a href="https://docs.aws.amazon.com/apigateway/latest/developerguide/apigateway-websocket-api-selection-expressions.html#apigateway-websocket-api-apikey-selection-expressions">Selection Expressions</a> for a list of expressions and each expression's associated selection key type.