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You don't necessarily need to opt for Device Co op to use Profile Merge rules.
Profile Merge rules allow you to define rules when to merge a user's profile with all their linked devices. So, it let's you create your own first party data device graph.
By default you get these device options: No device profile, Current Device, and Profile Link Device Graph (upto 100 linked devices).
You can additionally add few other device graph options for example Tapad etc. and if you are operating in North America region, then you can also opt for Adobe's device graph: Device Co-Op.