A multiple is any person, either one with DID or not, who habitually shares their mind and body with other persons/personalities/entities.

A mental collective is new word. Previously all multiples, both DID and non-DID used the term system.

A mental collective is distinct from a system in that a system is composed of alters- persons created by the mind of the original single person as a response trauma. 

A mental collective is composed of persons who are not alters. They’re referred to as headmates. Unlike alters, headmates are not created by trauma, and rather are created by, or enter into the mind in different ways. 

A natural multiple is a multiple whose experience is non-disordered. That is, that having multiple people share their body is not a result of trauma, and does not cause undue social problems or mental stress.

Please let me know if there’s any more I can explain!