Frequency: Once or twice per sprint (Not mandatory)
Meeting length: 2 - 4 hours
Backlog refinement (formerly known as backlog grooming) is when the product owner and some, or all, of the rest of the team review items on the backlog to ensure the backlog contains the appropriate items, that they are prioritized, and that the items at the top of the backlog are ready for delivery.
Items should be discussed and prioritized based on customer / market needs.
The issues should be completed within a sprint, thus, if there are any issues that are not
Some of the issues may be deferred due to change of requirements or necessity of that feature, thus, cleanup of the backlog items can be done here.
This has its' own meeting, attendees should not discuss the sprints that these items will be developed.
This meeting is to clean the backlog, completed items are not in scope of this meetings.
These can be done during retrospective, this has nothing to do with the grooming meetings.