In general, an object interaction is usually a very complex set of activities between two object sets that is abstracted for purposes of documentation. Communication itself is a very complex activity and is very difficult to model. Labels are merely identifiers of general classes of activities and communication. Activity Parameters and Messages Many interactions between objects involve the exchange of information. A computer sends a document to a printer and a printer will send back an error message if it encounters problems during printing.
Students ask teachers questions and receive answers to their questions.
Likewise, any activity can have one or more objects sent as parameters to the receiving object set and that same activity can return one or several objects to the sending object set. Parameters are listed inside parentheses. The parameters before the arrow are sent to the receiving object set and the parameters after the arrow are returned to the sending object set. The above form might give the impression that an interaction is like a function. However, an interaction does not have to have any parameters or return parameters.