About CNGroup.identifier

It contains speculation because it is not mentioned in the document, but :ABGroup suffix is given when it is a locally created group.

The Containers and Groups diagram in Contacts framework makes it easy to see, but there can be multiple CNContact data sources (=account) that you can retrieve from your contact book.

A user may have contacts in their device's local account and/or server accounts configured to sync contacts.Each account has at least one container of contacts.A contact can be in only one container.

Private Translation: Users can have device local, one of the server accounts on which synchronization is configured, and multiple contact books.Each account has at least one container.The contact book may be a single container.

Contact book data is a collection of various data associated with your device, such as local devices, iCloud, Gmail, Facebook, etc.

An account has one or more collections called CNContainers.

A group is a set of contacts with a container. Not all accounts support groups and some accounts support groups.

Personal Translation:Group is a set of contact books in a container.Not all accounts support groups, and some accounts support subgroups.

CNGroup represents this CNContainer collection of internal contact books

For this reason,

• Name of the group (name)
• group identifier (identifier)
• Containers to store (identifier in CNContainer)

I have three pieces of information, but when I store groups in local data, there is no data source or container, so I think I distinguish them by suffix in :ABGroup.

let saveRequest=CNSaveRequest()
let group = CNMutableGroup()
group.name = "test group"
saveRequest.add(group, toContainerWithIdentifier:nil)
try!self.store.execute(saveRequest)

Thus, if saveRequest.add(_, toContainerWithIdentifier:) specifies nil, it is saved locally and :ABGroup suffix is granted.

When reading externally linked group information, or

let contact=/*Externally linked contacts you want to include in the group*/
let container=try self.store.containers(matching:CNContainer.predicateForContainerOfContact(withIdentifier:contact.identifier))
let containerIdentifier = container.identifier

ABGroup suffix was not given when toContainerWithIdentifier: was retrieved from CNContact other than local data and passed it to toContainerWithIdentifier: to create a group.