USAN stands for Universal Simple Account Number and is an address that can be used on Fusion as an alternative to public keys, which are a bit tough to memorize or even copy number by number. USANs are much shorter and in decimal format, which most people are more used to than HEX. Basically it's an easy to remember number, like a phone number that can be uniquely yours and which people can send you crypto at.
Just like TL and QS, USANs exist as part of the core protocol function of Fusion and can be generated by the help of myfusionwallet and whallet. It's also likely to serve an an important function in Chainge which has aimed to bring the social element into crypto even before it launched. So I'm pretty sure that in Chainge it'll be possible to save a list of contacts and their respective USANs.
USANs only exist on the Fusion blockchain, but since you can bring other cypto assets to Fusion, it could eventually serve as a general purpose crypto address, as long as you're willing to keep every crypto assets on Fusion.
Noteworthy historic mention, is an interesting event which took place on Twitter in 2020. This was that hackers hacked the official Twitter profiles of famous people such as Bill Gates, Joe Biden, Jeff Bezos, Barrack Obama and Elon Musk, posting up a bitcoin address in their profiles and asking for money. As a result of this, public key HEX-type crypto addresses were banned from being written on Twitter, making it impossible for the same thing to happen again. What's interesting about this is of course that a USAN consisting of a simple plain number, could never be banned from public places in the same sense. Thus it might be an important key for mass adoption, in an unexpected way.
After creation USANs exist on Fusion kind of like an NFT and they can also be traded on the Quantum Swap Market. An extra simple and intuitive USAN could thus actually become worth some money for the same reasons that a popular domain name could be worth some money. The USAN market has though as of yet been quite limited, but who know what will happen once better and more intuitive wallets support them? It could "Chainge".