Service Broker uses a transmission queue as a holding area for messages. Each database contains a separate transmission queue. The transmission queue is stored in the primary filegroup for the database.
In cases where both sides of the conversation are in the same instance, Service Broker may optimize message delivery by placing the message directly on the destination queue.
If Service Broker message delivery is not active in the database that sends the message, or the destination database is unavailable, messages remain in the transmission queue for the database that sent the message.
If the destination database is available, but the destination queue is unavailable, the message is held in the transmission queue for the database that contains the destination service.