Where you register an application, get your api_id and api_hash, and manage your GROM account from a browser.
Register a client application to receive an api_id and api_hash.
Every device signed into your account, and a way to terminate any of them.
Erase your GROM account and everything attached to it. Permanent.
When the portal opens, the flow is deliberately not the usual one:
The code goes to the app rather than to your SIM on purpose. SMS codes can be intercepted by taking over a phone number, which is a routine attack rather than a theoretical one; a code delivered inside an already-authenticated app cannot be captured that way. The trade-off is real, though: you need GROM installed and signed in somewhere before you can use the portal at all.
The portal session is separate from your app sessions. Signing out here will not sign you out of GROM, and signing out of GROM will not end a portal session.