@NASCARaddicted--if you are concerned about it, use UUID unique identifier instead. Sigil generates a tag that looks like this:
<dc:identifier id="BookId"
opf:scheme="UUID">urn:uuid:29d0d5ab-896f-4667-8e4f-38c5eca2d78f</dc:identifier>
But as far as I know, you can replace that long alpha-numeric string with anything you choose. For example:
NASCAR-book001-15-03-2014
edit to add-- See discussion at this thread:
https://www.mobileread.com/forums/sho...d.php?t=213906