An end is a termination point of the flow represented in the diagram.
You can create several ends within the same diagram, if you want to show divergent end cases, like errors scenarios:
If there is no end, the diagram contains an endless process. However, a decomposed process must always contain at least one end.
You should not use the same end in more than one diagram. You are not allowed to use shortcuts of ends, but graphical synonyms are permitted.