STEMMA has the capability to represent relationships between entities of different types. This is done using Properties in a <SourceLnk> element of an Event, where the extracted and summarised forms are assembled. However, person-to-person relationships are far more common than the others, and attempting to decide whether something is a group-relative relationship or an event-relative role may be a subjective decision.
The Person Property called Relationship has predefined values for biological relationships, and a number of other predefined ones for common non-biological cases. The biological ones may be recognised and acted upon by software, but all others are interpreted blindly as 'relationships'. If a researcher determined that some of those relationships were more binding, or more significant, then a Group entity could be used to collect them together and document why. A typical example might be a “family unit” since no program can infer it all by itself.
The following person-to-person relationships must be relative to a selected person reference and so the Relationship Property has a data-type of PersonEL. That means it must have a Key attribute identifying the selected person reference, and a list of one-of-more relationship terms.
For instance, the following would be read as (John).Wife.Friend:
<Property Name=’Relationship’ Key=’pJohn’ Value=’Wife.Friend’>
Friend of John’s wife
Note that the “Relation to Head of Family” column in the census of England and Wales is overloaded by representing both event roles and personal relationships. A more detailed discussion can be found at Role of the Role.