This document discusses different schema designs for common use cases in MongoDB. It presents four cases: (1) modeling a message inbox, (2) retaining historical data within limits, (3) storing variable attributes efficiently, and (4) looking up users by multiple identities. For each case, it analyzes different modeling approaches, considering factors like query performance, write performance, and whether indexes can be used. The goal is to help designers choose an optimal schema based on their application's access patterns and scale requirements.