A typical Instance Identifier looks like this:
<id root="18.104.22.168.22.214.171.124.1.1" extension="123456" />
These identifiers are intended to be globally unique. To ensure uniqueness, the “root” part of the identifier is an Object Identifier (OID) that is managed by OID management agency. (I’m not entirely sure who that is, but someone does it. It may be COSIRA). The extension is an id that is relatively unique — that is, unique in the context of the OID.