Giving a variable definition scope allows the variable to be referenced not only at the hierarchical level of the diagram at which it was defined, but also at all the levels beneath it. For example, if compound block A contains a definition scope variable, all the sublevels in compound block A can use the variable.
By using definition-scoped variables, you can copy or add subsystems to an existing diagram without breaking or misdirecting references.
To give a variable definition scope, preface its name with two colon (::) characters.