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MoSCoW rule
When working with stories from a product backlog especially during release planning, Write all the epic stories ( the main use cases ) and instead of stank ranking them numerically, apply the
- Must Have,
- Should Have
- Could Have
- Wont Have
rule to each story . i.e. ask the product manager to write a M S C or W in front of every story. Sometimes product owners find it tough to apply numbers but a grouping like this is much easier for a first pass.
Agile Practitioners Guide 
This is a rule that gets mention in many books. Check out Agile retrospectives by Diana Larson
Hello again Vibhu,
This is really an interesting rule, where does it comes from ?
Thanks for sharing,
Philippe