A start is a starting point of the flow represented in the diagram, and an end is a termination point of the flow.
You can create several ends within the same diagram to show divergent end cases, such as error scenarios:
You should not use the same start or end in two diagrams, and you cannot create shortcuts of starts or ends.
If there is no end, the diagram contains an endless activity. However, a decomposed activity must always contain at least one end.