The Product Backlog is the reference for the scope and the features of the final product. Therefore, it contains the requirements as defined by the Product Owner as well as bug fixes, technical requirements (such as the setup of a test server), and issues regarding other relevant topics, e.g., documentation and training material.

The process of defining backlog items is ongoing and continuous. While the Sprint Backlog is immutable during a sprint (unless the Product Owner cancels it), the Product Backlog can be changed at any point in time. Whenever the Product Owner has a new requirement, a new bug report has come in, or the Scrum Team decides that a technical issue must be dealt with, the corresponding item can go on the Backlog. Once it's there, it can be prioritised, estimated, and finally scheduled for one of the sprints.