An object node is the association of an object (instance of a class) and a state. It represents an object in a particular state.
Its symbol is a rectangle as shown below:
An object node can be created in the following diagrams:
The same object can evolve after several actions defined by activities, have been executed. For example, a document can evolve from the state initial, to draft, to reviewed, and finally turn into a state approved.
You can draw flows from an activity to an object node and inversely:
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A flow from an activity to an object node - means that the execution of the activity puts the object in a specific state. It represents the result of an activity
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A flow from an object node to an activity - means that the activity uses this specific state in its execution. It represents a data flow between them
When an activity puts an object in a state and this object is immediately reused by another activity, it shows a transition between two activities with some data exchange, the object node representing the data exchange.
For example, the object nodes Order approved and Invoice edited, are linked to the classes Order and Invoice, which are represented in a separate class diagram:
