Actions operate on objects and are used by objects. These
objects either initiate an action or are used by an action. Normally,
actions specify calls sent between the object owning the activity
diagram (which initiates actions) and the objects that are the targets
of the actions. An object node represents an object passed
from the output of the action for one state to the input of the action
for another state.
Note: An object node can only
be a leaf element; its meaning cannot contain other elements.
As with other states, only one object node can have
the same name in the same parent state.