Like the Verification queue, the Exception queue lists tasks that need to be actioned, except that these tasks are separated out into this queue as some exception has occurred.

This could be for a range of different reasons, but this most commonly happens if a document got completed in Verification when it still contains rule errors, or if someone sent the document to Exceptions with a comment (e.g. vendor does not exist). 

If, however, it is not immediately clear why the document is in Exception, please log a ticket with support to investigate and advise.