RGB++ seems to use the Nervos Network blockchain (CKB), a "PoW & UTXO based chain", which is the first "+", the second "+" being Lightning. So it seems to be basically an altcoin variant of RGB, but it is not independent from Bitcoin but uses
both Bitcoin and the CKB chain. It seems not to be an
official RFC of this blockchain but an independent application.
I've found a quite good article
here at Coinlive explaining not only the differences between RGB and RGB++ but also gives some examples how RGB (the original) works, for example how tokens are transfered over RGB, which is something I didn't found that much easy-to-understand info about (even if it's clear that the article's writer's native language isn't English but that's not a problem).
The main issue RGB++ aims to solve is the "data island" problem. This means that as in RGB almost everything happens offchain (with the exception of ownership transfers), the participants of e.g. a token do not necessarily know the token's global state, but they know only what they store on their own hardware (which is basically knowledge about a partial state derivered from the knowledge of all other users which have interacted with you with a specific RGB contract, such as those having transferred tokens to you).
The idea of RGB++ is thus to store the global state of the token, or at least a part of it, on the CKB chain.
The question I ask however is: isn't the added value of this system compared to a completely altcoin-driven token system quite low? Yes, it benefits from Bitcoin's security for ownership transfers, but it also seem to depend heavily on a less secure blockchain's consensus - and they could run into an inconsistent state, if for example the CKB chain is attacked, but many users have stored parts of the "correct" state (i.e. based on "honest" transactions). From my interpretation the original RGB is "cleaner", because the responsibility to store the correct state is always on the users' side. But again, I'm not an expert on that topic, would really like to hear some expert opinion