CID stands for “Carrier ID”, and simply tells for what region/network/carrier your phone was intended for.

I’ve seen many times phones that’s been sold as “unlocked” etc, which turns out to be just sim unlocked but still branded to a network.

So? That means that updates goes via the network in question, adding weeks to the update.  Just to understand why and how, and if you imported your phone you will see where it comes from.

The Easy way

Install CID getter from Play store. Install it, and it will tell you the CID in red letters high up in the app.

The Nerdy way

Follow THIS, but enter the following to just see CID.

fastboot getvar cid