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).
To resubmit a document for processing, open the task in the Exceptions queue and click on Send Task to Stage button. In the dialog select Stage pre-Verification and click SEND.
After a few seconds, the document will appear in the Verification queue as normal.