When defining a diagram type, you can also set the multiplicity of each element type.
This means you can say, for example,
•only one element of a type can be on the diagram.
•that a maximum of 10 elements of another type are allowed
•that the modeller must have at least one element of another type.
This can be used to control more of how a user uses a diagram.
To set the multiplicities, just add a normal UML multiplicity to the element-type end of the appropriate connector.
For example, in the 'Diagram #3' process diagram, we can say there MUST be one, and only one, Start Event.