The ID page is used to increment the actual ID if the naming format is numeric. The ID page and the Start page may or may not be the same. If you are using Bates numbering, only the first Bates document will match the ID page.
example:
For a three page Bates numbered document:
DOC0001 ID Page = 1 Start Page = 1
DOC0002 ID Page = 2 Start Page = 2
DOC0003 ID Page = 3 Start Page = 3
For the next Bates numbered document:
DOC0004 ID Page = 4 Start Page = 1
DOC0005 ID Page = 5 Start Page = 2
DOC0006 ID Page = 6 Start Page = 3
Note - you may start at any ID page number you wish as long as it does not create duplicate values. This may be necessary when inserting pages after a re-number.