We asked that question earlier this year, and already have an answer. A parcel needs to be flagged if it contains non PG content.

Not all parcels in mature sims actually *have* mature content (for example, mine don't) and thus don't need to be flagged.

So - a mature sim *can* (but does not have to) contain parcels that may contain mature content or activities. A PG sim can *never* have a parcel with mature content or activities. Any parcel with mature content or activities must be flagged.