Yes it will be the unique ID. Whenever you add an item to the list a new ID will be assigned to it. There is nothing like reusing ID.
When the Document ID service is first enabled, new documents in the site collection will automatically be assigned new IDs. A timer job is scheduled to assign IDs to documents already in the site collection. Also, when the service is enabled, a new column is automatically added to the Document and Document Set content types. The Document ID column displays the ID assigned to an item.
In order enable the Document ID feature you must be a site collection administrator.
- Go to the top-level site collection.
- Click Site Actions and then click Site Settings.
- Under Site Collection Administration, click Site collection features.
- Next to Document ID Service click Activate. An Active icon appears next to the item and the feature is activated for the current site collection.
Document IDs consist of two parts, a prefix (that is randomly generated per site collection by default) and two sets of numbers. An example Document ID looks like this:
J2W3DN6QF6XW-2-10. The first number is the ID of the list that the document was first created in and the second number is the ID of the item in that list. The prefix can be specified per site collection and can easily be changed from the Document ID settings page.