Label IDs are used to organize labels in different groups (e.g. all labels with a special kind of support values for the tree nodes). Assigning labels IDs has the advantage that formats (e.g. font family and color) of all labels in the document belonging to this group can be changed in one step.
Label IDs have to consist only of letters, digits and "_" (e.g. "id1", "123" or "_a" would be valid IDs, whereas "abc$" would not).
If you have inserted a new label in a document you can can assign an ID by clicking selecting "Change label ID..." form the edit menu when the accordant label is selected.