What you need to know
Embedding related data can reduce reads when it is usually displayed together. Referencing separates records that change independently or grow without a predictable bound.
How to apply it
Start from the most common query, set size limits, and consider update patterns and ownership. Embed small bounded details; reference entities shared across many records or collections.
Common mistake to avoid
Do not embed unlimited activity history inside one document. Document size and update contention can become a problem as the record grows.