Object Flows are used in Activity diagrams and StateMachine diagrams. When used in an Activity diagram, an Object Flow connects two elements, with specific data passing through it, modeling an active transition. To view sample Activity diagrams using Object Flows, see the Object Flows in Activity Diagrams topic.
In StateMachine diagrams, an Object Flow is a specification of a state flow or transition. It implies the passing of an Object instance between elements at run-time.
You can insert an Object Flow from the 'State' or 'Activity' pages of the Toolbox, or from the drop-down list of all relationships located in the header toolbar. You can also modify a transition connection to an Object Flow by selecting the 'ObjectFlow' checkbox on the connection 'Properties' dialog.
See the Control Flow topic for information on setting up Guards and Weights on Object Flows.
OMG UML Specification:
The OMG Unified Modeling Language specification, (v2.5.1, p.376) states:
An ObjectFlow is an ActivityEdge that can have object tokens passing along it. ObjectFlows model the flow of values between ObjectNodes. Tokens are offered to the target ActivityNode in the same order as they are offered from the source. If multiple tokens are offered at the same time, then the tokens are offered in the same order as if they had been offered one at a time from the source. If the source is an ObjectNode with an ordering specified, then tokens from the source are offered to the ObjectFlow in that order and, consequently, are offered from the ObjectFlow to the target in the same order