They can be modeled only for the source pattern, that is, they can only have outgoing connections.
Constants are used for identification, mapping attributes, or they represent values in certain cases of conditional mapping.
Procedure
First, you need to model the source pattern of a section of the model.
Click Constant and place the constant in the mapping model.
Model the corresponding connections.
You have modeled a constant. If you used a constant from the Constants tab, an occurrence copy is created in the mapping model, that is, if you change the constant it is changed in all mapping models in which it is used.