Whatsapp Business API: In which case does the contacts field of an inbound message contain more than one contact?

The WhatsApp documentation states:

The contacts object provides all the information about the contact.

which also contains the wa_id which is

The WhatsApp ID of the contact.

Each inbound message has the contacts and messages fields, but I fail to understand why there would be multiple elements in the contacts field. As I understand it the wa_id from the contacts object is the sender's id which I should use to send messages back to the user who did send these messages. Also, none of the examples in the docs has more than one element in the contacts array.

So far I`m assuming that structure was in use when the business API still supported groups but I did not find anything supporting this assumption.

In which case does the contacts array contain multiple contacts?