ERC3525 is considered a final EIP standard since September 2022, built by people of the Solv protocol. First of all it's compatible to ERC-721 so from the perspective of marketplaces and user wallets it behaves like NFTs. It adds another "value unit" that's used to represent NFT fractions or a certain weight or value of each token. Tokens that are minted inside the same "slot" are considered fungible and share identical metadata (tokenURI responses). Holders of an SFT can send value units to other SFTs on the same slot, i.e. ERC3525 tokens can act as a "wallet" that represents value and transfer that value to other users holding tokens of the same slot. When an user transfers value units from their SFT to an address that doesn't hold an SFT on the same slot yet, the default implementation mints a new SFT for the recipient.
At molecule.to we've deeply investigated the ins and outs of this new primitive as a foundation for our IP-NFTs that represent real world intellectual property and its usage rights. Also, we're in touch with Protocol Labs' hypercerts team who has built a pretty decent prototype of the ERC3525 protocol (and currently iterates on it). Now we're wondering if there are any plans to adapt this standard into OZ's base library any time soon or if you have any (in)official opinions on ERC3525. Here's an end user oriented website that illustrates its feature set and rationale.