What product backlog items should be refined?
Any product backlog item should be refined as and when additional information is known about it. However, priority should be given to refining the product backlog items that are likely to be taken into the next sprint. This will ensure that enough information is known about each item to allow for it to be sized, for the plan to be generated during sprint planning as to how the selected product backlog item can be delivered, and to allow for the team to commit to delivering that and other items in the next sprint.