In Scrum, a PBI (Product Backlog Item) is a single element of work that exists in the product backlog. PBIs can include user stories, epics, specifications, bugs, or change requirements. The product backlog is a prioritized list of features and enhancements that the development team plans to implement in future releases of the product. PBIs must be small enough increments of work to be completed during a single sprint. The Product Owner of an Agile team compiles and prioritizes the product backlog, putting the most urgent or important PBIs at the top. Developers pull the highest-priority PBIs from the backlog to work on, either by iteration in a Scrum environment or continually in a Kanban environment. PBIs are what get refined during the backlog refinement session, and if one is too large, it may be broken down into smaller PBIs.