UML14 — Activities operate by and on objects

Summary: The notation document says regarding object flow:

Activities operate by and on objects. Two kinds of relationships
can be shown: 1) The kinds of objects that have primary
responsibility for performing an action and 2) the other objects
whose values are used or determined by the action. These are
modeled as messages sent between the object owning the activity
model and the objects that are input or output by the actions in
the model. [section 3.80.1, p 130 Notation 1.2]

Presumably the activity model doesn"t tell the objects being passed

Clarified in UML 1.3