An establishment is the official collective term for commercial establishments in which sexual services are offered - regardless of whether they call themselves a "salon", "brothel", "massage institute" or "bar". What is important is the actual activity, not the flagship.
Establishments are subject to cantonal licensing law. In Zurich, Basel, Bern, etc., an express patent approval is required. The authorities regularly check: fire safety, minimum age, residence status of sex workers, voluntary nature of the activity.