The general consensus is that:
- Product backlog Item is something that can be delivered in a single sprint.
- Feature is something that can’t be delivered in a single sprint, but that can be delivered in a single release.
- Epic is something that transcends releases.
- Theme is a cross cutting concern.
Theme is generally implemented as a tag in TFS and VSTS.
This is a practice that is in line with Scrum Framework, Nexus Framework, and SAfE methodology.