The $ID value of an item may be used as a special designator for single items or groups.
For example:
$Name(1524673590) locates the $Name of the note whose $ID is 1524673590.$Name(1524673590;1524673572) locates the $Name of the two notes whose $IDs are listed.Note that this particular designator usage—with literal ID values—should be avoided whenever possible, as it is hard to read and thus confusing for the occasional user. However it is made available because in some circumstances it may be the only practical way to reference item(s) where relying on unique paths are impractical.
Indeed in complex code use, where lists of references to notes are passed around often (including in List or Set type variables), ID codes are much more robust to title or path mis-parsing due to the presences of slashes or parentheses in not names.
See also—notes linking to here: