The unique ID for a DocumentsHierarchyNode.
The documents hierarchy may contain multiple references to the same document, and the same document may be spread out over multiple hierarchy nodes. To disambiguate cases, operations may be targeted at a specific hierarchy node to ensure the cursor is placed in the correct instance of the document. Operations that modify the hierarchy may also use this information to determine the instance corresponding to the current selection.
In cases where the hierarchy node is not specified, Fonto prefers the sheet frame that most recently contained the cursor, and otherwise uses the first instance in the hierarchy.
When the hierarchy is rebuilt as a result of operations that modify it, or as a result of documents being reloaded, Fonto will try to reassign IDs for comparable hierarchy nodes (i.e., same source node, or same target) from the previous hierarchy. This way, these IDs provide a convenient handle to minimize updates of derived data and UI in response to hierarchy changes.