A MetroID is required to generate a MetroGnomo timestamp. Every time you provide an email address a new MetroID will be sent to your inbox. This can be activated by clicking the link in the email. You can retrieve your MetroID’s in the future using your email address.
Why obtain more than one MetroID? Different MetroID’s could be used for different clients, or for added anonymity. Alternatively, a MetroID can be reused to ease the process of locating your ledger entries.