they are not unique to be able to group todos (in this case it is close to +context meaning)
where the number of a todo is used, ID is good to go as well (maybe with optional id: prefix when an ID is a number. For alphanumeric ID it can be auto-detected)
reserve parent: for referring to another ID. It may be useful to keep todo hierarchy. And when closing the main todo, all todos that refers to it are closed automatically as well. And closing all children may auto-close the main todo
It may be convenient to use IDs for todos:
+contextmeaning)id:prefix when an ID is a number. For alphanumeric ID it can be auto-detected)parent:for referring to another ID. It may be useful to keep todo hierarchy. And when closing the main todo, all todos that refers to it are closed automatically as well. And closing all children may auto-close the main todo