Hello,
the Source and the Target refer to the units connected by the Link. During the modeling, when you draw a Link you define the source unit and the target unit.
You can edit the Link Parameters in the Parameters Coupling dialog. To open the dialog double-click on the Link, or select the Link and click on the button next to the ""Coupling"" item in the Properties Panel. In this dialog you can couple the parameters of the source unit with the ones of the target unit. To set a coupling, select a parameter of the source unit from the drop-down, or write a constant value for the corresponding target parameter.
The source and the target parameters depends on the units (source parameters = output parameters of the source unit; target parameters = input parameters of the target unit). For example, the output parameters of a Data Unit are all the Attributes of the selected Entity, while the input parameters of a Data Unit consists of all the Conditions defined on the Unit.
You can see the details of a Link Parameter in Properties Panel: expand the Link in the
Outline View -> select the Link Parameter.
For more details / examples: