.ItemByID() how to obtain ID

The ID is an internal unique identifier for the table (the ESTK Help describes it as "The unique ID of the Table." -- kind of summarizes it). It's guaranteed to be unique for all tables throughout your entire document -- InDesign needs it to be for every single object to be able to put your file together from its bits.

I can sort of imagine how it works: what if you rename a paragraph style? If the styles were applied "by name", ID would have to scan the entire document for that name and change it. What if you move a paragraph style around in its palette? If the styles were applied by using their index number (the index being their position in the palette), ID again would need to re-scan the document. Instead, all styles are applied by UniqueID. All InDesign has to do now is making sure it does not re-use an ID when you create a new style. The Unique ID will stay the same, whatever you do with the style. And if there is no style with some ID in the paragraph style list, it does not exist. Period.

.. Totally made this up, of course, but it does sound good doesn't it!?

The ID is "hidden" -- not available in the User Interface, and read-only for scripting purposes. If I would be prone to guessing, I'd say it's even read-only in plugins, except (maybe) at the very deepest core.

This is a Javascript that shows the ID for the table your cursor is in. However, interesting as that may sound, logically, you might not need it. If you can read the ID of a table, you already have to have a valid handle on it. And if you do, you don't have to use ItemById ...

I think it could be useful to store the table IDs somewhere, do something else, then do something with the stored IDs. But you can always store direct pointers to the tables themselves ... (And -- again, guessing -- perhaps the scripting interface store these internally by referring to their ID.)

Perhaps you could better use the zero-based index (or, in the case of VBS, 1-based) of the tables in your story. The first index always will point to the first table, and so on all the way to the end of the story.

if (app.selection.length != 1)
{
     alert ("Please select more or less than this. Preferably a TABLE!");
     exit(0);
}
table = app.selection[0];
if (table.hasOwnProperty("baseline"))
     table = table.parent;
if (table.constructor.name == "Cell")
     table = table.parent;
if (table.constructor.name == "Row")
     table = table.parent;
if (table.constructor.name != "Table")
{
     alert ("Ah. Not in a table, are we?");
     exit(0);
}
alert ("Table Index: "+table.index+"\nTable Unique ID: "+table.id);