Resolution
Seems that it is always set to 'Normal'
Currently none of the core code in Access Governance Suite sets this to anything other than "normal". If a customer has some custom code that generates work items, they could change this field to indicate a severity. But Out of the Box there is nothing special about it.
If you selct the Advanced Search option on the Dashboard it does display as a Filter with the options of Normal Low or High, so if those work items did have a Severity set this might be useful.