Clients can have multiple contacts, which is great for tracking parents, siblings, entire families, teachers, employers etc. Contacts can have appointments and invoices too as well as receive appointment reminders for the client's appointments.
You are also able to add an existing client as a contact. For example, if two existing clients are partners or a parent and child you are able to resemble this relationship by adding one of the clients as a contact for the other client.
You can search for contacts via People > Clients and using the 'Contacts' toggle