Test 1: I created PBI through web access just providing Name and click add in Backlog.
Test 2: I create PBI with web access going to Queries-->New-->PBI
Test 3: Changed Microsoft.VSTS.Common.BacklogPriority manually.
See how it behaves..and hope you will have a clear understanding of Microsoft.VSTS.Common.BacklogPriority. In terms of usage you can go through this MSDN article.