Category: semi-fungible token standard / slot-and-value primitive / tokenized-asset middleware
Summary: ERC-3525 is a lower-layer token standard for assets that are neither purely fungible nor purely non-fungible. Its core move is the <ID, SLOT, VALUE> model: each token keeps a unique ERC-721-style identity, but tokens that share a slot can also hold fungible value that can be partially transferred, combined, or reallocated. That makes ERC-3525 analytically useful because it exposes a distinct control surface between ERC-20 balances and ERC-721 items. Instead of forcing a system to choose between fully interchangeable units and fully unique objects, it lets unique tokens carry quantitative balances and lets value move between them when they belong to the same slot. The result is a clean comparison class for tokenized bonds, vesting instruments, structured products, and any system where identity and quantity both matter.
What it does:
Standardizes the <ID, SLOT, VALUE> structure, where token identity is unique but value is fungible across tokens sharing the same slot
Adds transferFrom(fromTokenId, toTokenId, value) so value can move directly between two tokens with the same slot rather than only between addresses
Adds transferFrom(fromTokenId, to, value) so part of one token’s value can be reassigned to a recipient address through a same-slot token representation
Introduces value-level approvals and optional slot-level approvals, making delegated quantitative control more granular than ordinary ERC-721 approvals
Keeps ERC-721 compatibility for ownership and transfer while adding optional slot enumeration and receiver interfaces for the semi-fungible layer
Makes decimals, value balances, slot identity, and value-transfer events first-class instead of burying them inside custom wrapper contracts
Key claims:
The most important mechanism insight is that ERC-3525 does not just add fractions to NFTs. It makes identity and quantity coexist at the standard level, which is a different primitive from both ERC-20 and ERC-721.
The slot abstraction is the real control plane. It defines where value is fungible, which means it quietly determines what counts as the same economic instrument even when token IDs remain unique.
Token-to-token value transfer is more revealing than the marketing label semi-fungible. It treats tokens as containers or accounts whose balances can be reallocated, not merely as indivisible collectibles with optional fractions.
The reference implementation README usefully pushes the standard beyond the EIP’s abstract wording: it emphasizes fractionalization, combination, computability, token nesting, programmable lockups, and more complex financial logic. That helps explain why ERC-3525 became attractive for structured products and SFT-style tokenization rather than as a generic NFT extension.
ERC-3525’s strongest comparison value in this corpus is against standards like ERC-1410 and ERC-3475. ERC-1410 segments rights by partition inside an aggregate balance, ERC-3475 segments obligations by class and nonce, and ERC-3525 segments fungibility by slot while preserving unique token identities.
ERC-3525 belongs in the active corpus because it gives the tokenization cluster a reusable lower-layer comparison point for semi-fungibility, tokenized balances, and quantity-carrying NFTs before richer compliance, issuer-identity, or admin-control systems are layered on top.
Whitepaper: ERC-3525 has an associated SFT white paper in addition to the canonical EIP and official reference implementation materials; see ../../whitepapers/erc-3525-primary-sources-2026-05-12.md.