This is necessary when you want to represent the same subject at different positions in the diagram. But, nevertheless, in most cases, the relationship between subjects and representations is one-one. For this reason, subjects and shapes can be identified most of the time. A TCM diagram is empty after creation. You can build it up by interactively drawing it with the TCM user interface, node by node, edge by edge.
Most constraints are specific for the particular diagram technique supported by the editor.
The constraints that are checked, are classified according to how TCM deals with possible violations. These are constraints which are built-in and which can never be violated because there is no command in the user interface to achieve that. These are constraints that are immediately enforced by TCM if that is possible. When you perform a command that would violate a constraint that is immediately enforced, this command is rejected immediately by TCM and a pop-up window with an error message is displayed. These are constraints that can be violated.