# USD0 Savings (sUSD0)

sUSD0 is the savings token for USD0, implemented as a **non-rebasing ERC-4626 vault**. It turns USD0 from a settlement stablecoin into a yield-accruing savings instrument that is **composable, transparent, and fully on-chain**.

Users deposit USD0 into a permissionless vault and receive sUSD0 shares in return. **sUSD0 balances do not rebase**; instead, value accrues through an **exchange rate that increases over time**. As a result, redeeming sUSD0 returns **more USD0 than initially deposited** (assuming positive yield over the holding period).

Yield originates from the protocol’s underlying collateral stack, sovereign US Treasury Bills and other short-term yield-bearing assets, and is passed through to sUSD0 holders via the vault’s exchange-rate mechanism.

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## Key Facts

* **Underlying asset:** USD0, the RWA-backed stablecoin at the core of Usual Protocol
* **Standard:** ERC-4626 vault (**non-rebasing**, accrual via exchange rate)
* **Access:** Permissionless; no KYC/KYB requirement
* **Yield source:** Sovereign T-Bills and overnight reverse repos backing USD0 collateral
* **Yield distribution:** Controlled via on-chain parameters; the rate may vary with market conditions and DAO decisions
* **Redemption math:** `USD0 out = sUSD0 × exchangeRate` (*t* ≥ 1)
* **Contract address (Ethereum):** `0xd861bE82dEe3223CFBEd160791f6550b0704D406`

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## How sUSD0 Works

1. **Deposit** USD0 into the sUSD0 ERC-4626 vault.
2. **Receive** sUSD0 shares representing a proportional claim on the vault’s total assets.
3. **Accrue yield passively** as the **sUSD0↔USD0 exchange rate increases** over time.
4. **Redeem anytime** to unwrap sUSD0 and receive USD0 at the current exchange rate.

Because sUSD0 follows the ERC-4626 standard, it is straightforward to integrate across DeFi. Any protocol that supports ERC-4626 can generally support sUSD0 **without custom adapter logic**, simplifying accounting and composability.

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## Yield Source and Collateral Stack

The yield that accrues to sUSD0 traces back to the DAO revenue. 

### How yield reaches sUSD0 holders

The protocol captures yield generated by the underlying collateral and routes a portion of it to sUSD0 holders by increasing the vault’s **exchange rate** over time. Distribution parameters—including how much yield is allocated to sUSD0 and how quickly it accrues—are adjustable via **USUAL governance**.

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## sUSD0 vs. USD0 (At a Glance)

| Feature            | USD0                          | sUSD0                                  |
| ------------------ | ----------------------------- | -------------------------------------- |
| **Purpose**        | Settlement asset              | Savings instrument                     |
| **Value behavior** | Pegged 1:1 to USD             | Grows via exchange rate                |
| **Access**         | Free mint / redemption        | Wrap / unwrap via ERC-4626 vault       |
| **Use cases**      | Payments, collateral, trading | Yield generation, DeFi integration     |
| **Design**         | Rebase-free stablecoin        | ERC-4626 wrapper (non-rebasing shares) |

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## sUSD0 vs. the Bond Product (bUSD0)

Usual Protocol offers two distinct ways to earn yield on USD0, each aligned to different risk profiles and time horizons:

| Feature                | sUSD0                                       | bUSD0                                                |
| ---------------------- | ------------------------------------------- | ---------------------------------------------------- |
| **Mechanism**          | ERC-4626 vault; exchange-rate accrual       | Liquid bond; daily USUAL token coupons               |
| **Lock-up**            | None — deposit and withdraw freely (3bips)  | Bond with maturity date (June 2028)                  |
| **Yield denomination** | USD0 (real yield)                           | USUAL tokens (alpha yield) or Base Yield Guarantee   |
| **Yield type**         | Predictable, T-Bill-based                   | Variable; dependent on USUAL token price             |
| **Early exit**         | Instant redemption at current exchange rate | Secondary-market sale or recombination with rt-bUSD0 |
| **DeFi composability** | High (ERC-4626 standard)                    | High (tradeable ERC-20; usable as collateral)        |

**sUSD0** is designed for users seeking **stable, predictable yield** denominated in USD0, with no lock-up and no direct exposure to token price volatility. **bUSD0** is designed for users willing to commit capital for potentially higher returns driven by USUAL emissions and bond discount dynamics.

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## Benefits

* **Predictable yield:** Transparent accrual via a rising exchange rate, backed by sovereign instruments
* **Stable collateral base:** Short-duration sovereign exposure (portfolio average duration **< 0.33 years**)
* **Non-rebasing design:** DeFi-friendly for pools, lending markets, and integrations that prefer fixed balances
* **Permissionless access:** Fully on-chain deposit and redemption—no intermediaries
* **Governance-aligned:** Yield distribution parameters adjustable via USUAL governance
* **Real yield in USD0:** Returns are denominated in USD0 and sourced from real-world fixed-income instruments (not primarily token emissions)

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## Rebasing Track

USD0 also supports a **rebasing mode**, where yield is distributed directly to **USD0 balances** at the “cash layer.”

This is part of Savings but lives on USD0 itself (not as a separate token). It is suitable for applications that prefer native balance growth over ERC-4626 wrappers, for example, wallets or payment interfaces where users expect their USD0 balance to increase automatically without wrapping into a vault token.

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## Contract Information

| Contract              | Address (Ethereum)                           |
| --------------------- | -------------------------------------------- |
| **sUSD0**             | `0xd861bE82dEe3223CFBEd160791f6550b0704D406` |
| **USD0 (underlying)** | `0x73A15FeD60Bf67631dC6cd7Bc5B6e8da8190aCF5` |