id
String id
This attribute is used to uniquely identify BPMN elements. The id is
REQUIRED if this element is referenced or intended to be referenced by
something else. If the element is not currently referenced and is never intended
to be referenced, the id MAY be omitted.
此属性用来唯一标示XML元素。
如果元素不被其它元素引用或者不为了被其它元素引用,此属性可以没有值。