Bark() is a unique Function which cannot be stored or equipped in the Transistor. It is exclusive to Fetches, Friends, and Luna.
Bark() is used both by hostile Fetches and by Friends spawned using Help(). It is their primary weapon. It can only be used at close range, but deals a large amount of damage, and Fetch 3.0s can deploy a Stunning Bark which is even more difficult to escape. Friends can choose to Bark() on their own or deploy multiple Bark()s on command in a planned Turn().
In the Sandbox, Luna's bark is solely used to bounce the Beachball and has no destructive effect.