The group element in TYPO3 makes it possible to create references to records from multiple tables in the system. This is especially useful (compared to the “select” type) when records are scattered over the page tree and require the Element Browser to be selected.
For database relations however, the
type='group' field is the right and a powerful choice especially if dealing
with lots of re-usable child records, and if type=’inline’ is not suitable.
The group field uses either CSV format to store uids of related records or intermediate mm table (in this case MM property is required).
You can read more on how data is structured in Stored data values chapter.