The actions an interface or component should perform when an event is received are specified within a Specifying behaviour.
Actions can be changing the value of a variable, invoking a function, generating another event or changing to another state and any combination.
In response to one event trigger, there can be different actions to be taken as long as they have non-overlapping guards. Guards are boolean expressions. The statement behind the guard is selected when the expression evaluates to true. The keyword 'otherwise' defines a catch-all guard which is true only when none of the other guard expressions in a list of guarded statements evaluates to true.
Note that the use of guards is reserved for action selection. The guards in combination with the trigger event determine which action is activated. Thus, the action is a (compound) statement inside an on event statement (optionally) behind an innermost guard.