The phases are simply based on priority.
1.Critical: Things that Mozilla has prioritised through their dashboard or bugs 1.Critical-FirefoxOS: FirefoxOS related high priority items, only do if you are shipping or planning on shipping FirefoxOS 1.Critical-Mobile: Firefox for Android related files 2.Good_to_have: Would be good to have these for your localisation 3.other: everything else 4.email: Emails generated for community participation