External switch, eg. connected to a bass drum pedal to sync sequencer to a band. In cast the sequencer has to sync to another source, it will keep using it's own local timer for determining schedule time of events. Send timing events to other ALSA sequencers, possibly running on another box and connected by a network. This opens the door for building sequencer clusters. The event transport mechanism is used for synchronization.
Clients can send and receive synchronization events.
The timer of the sequencer can be seen as an internal client.