Save - mStable Docs EN

Understanding Save
mStable assets are designed to generate a native interest rate for users who deposit mUSD or mBTC into the Save contract. This is achieved through the mStable smart contracts programmatically depositing the underlying assets to decentralised lending markets such as Compound or AAVE to generate yield. 90% of all revenue, including the value of token rewards from the lending platforms and swap and redemption fees generated through mStable, will be passed on to the users who deposit to the Save contract.
As interest and fees accrue, new mUSD or mBTC is minted and sent to the relevant Save contract. This means that the mStable asset is always close to being fully-backed by underlying assets, with very little deviation. Minor deviation can occur when the basket does not have an equal weight distribution, due to the nature of the bonding curve used to determine the prices for minting or redeeming assets. The parameters of this bonding curve are set in such a way that the deviation is smaller than 1%, 99% of the time. Weight limits ensure that the backing ratio will never drop below 95%.
This one-to-oneness in the value of mUSD or mBTC compared to the value of the assets in the underlying basket is in contrast with other Automated Market Maker LP tokens, which accrue swap fees implicitly and appreciate in value. This means that mStable assets can be used as pegged-value crypto asset in their own right, whereas LP tokens cannot. 
When a user opts in to receive interest by depositing their mUSD or mBTC tokens into the Save contract they will receive another token: an interest bearing imUSD or imBTC token. These interest bearing tokens will not have a one-to-one relationship to the value of the underlying peg, but will appreciate in value over time. They can be redeemed for the underlying mUSD or mBTC at any time, resulting in the user receiving their original deposit plus accrued interest. 
All savers are also given the option to participate in protocol governance by depositing their interest bearing assets into the "Vault". This is a place where savers can store their imUSD or imBTC to earn MTA rewards. As mStable is a collectively governed protocol, it is important that those who receive system revenue as yield have a say in protocol governance. The vault is designed to incentivise savers to become MTA Governors and participate in governance. Users who stake MTA to participate in protocol governance can increase the amount of MTA rewards that they receive through the Earning Power Multiplier.
Example
A basket with a total value of 1000 mUSD, comprised of equal parts DAI, USDC, USDT, and sUSD earns interest at the following APY derived from the mStable smart contracts autonomously lending deposits onto decentralised lending protocols, currently AAVE and Compound.
DAI - 5%
USDC - 4%
USDT - 3%
sUSD - 2%
A user deposits 100 mUSD into the Save contract, alongside others who have already deposited 400 mUSD. The Save contract contains a total of 500 mUSD (50% of mUSD supply), and over the course of 6 months accrues interest.
At the end of 6 months, assuming no underlying assets have breached their max weights, the basket is comprised of the following: 256.25 DAI, 255 USDC, 253.75 USDT, and 252.5 sUSD. In total, the basket has accrued 17.5 USD of value in interest at an average rate of 3.5% APY.
During this period, the platform also collects 10 mUSD in fees across swaps and redemption activity. This brings the total USD value accrued in the savings contract to 27.5 USD.
During this period, the protocol also received $10 worth of Compound tokens from lending on that protocol and $10 worth of AAVE tokens from lending on that protocol. These $20 worth of tokens are autonomously liquidated into mUSD, which brings the total USD value accrued in the savings contract to 47.5 USD.
Our user, having contributed one fifth of the total USD in the savings contract over 6 months, earns a corresponding 9.5 mUSD in interest.
NB - This example assumes most of the system to be static over this 6 month term. Realistically, the mStable system is a dynamic one, and returns will be influenced by multiple factors in the system changing on a regular basis, such as users minting and redeeming every day.
Using Save
Depositing assets to Save 
To deposit assets and start earning yield with Save, follow the steps below:
Navigate to the Save page on the mStable App on Ethereum or Polygon and select the Deposit tab on the form.
Choose whether you want to save with mUSD or mBTC
Deposit supported assets:
If you already have mUSD or mBTC, you can deposit this to the Save contract.
USDT, USDC, DAI and sUSD deposits will be used to mint mUSD .
GUSD, BUSD, or alUSD deposits will be used to swap for mUSD through the Feeder Pools.
renBTC, sBTC, WBTC deposits will be used to mint mBTC.
TBTC, HBTC deposits will be used to swap for mBTC through the Feeder Pools.
You can also deposit directly from ETH on Ethereum or MATIC on Polygon.
Select whether you would like to receive imUSD or imBTC in your wallet or deposit directly to the Vault to earn MTA rewards. Depositing directly to the vault can save you gas fees compared to saving and depositing to the Vault in separate transactions. Note that on the Polygon App, you will need to deposit first and then stake tokens in the Vault through the Stake tab.
If you opt to receive imUSD or imBTC, these tokens can be held or transferred and used as collateral in DeFi.
Withdrawing assets from Save
To withdraw assets from the Save Vault, simply select the Redeem tab on the form and select whether you would like to redeem imUSD or imBTC from you wallet or from the Vault. You can then choose an amount to withdraw and redeem for mUSD or mBTC which will reflect your original deposit plus accrued interest.
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Outline
Understanding Save
Example
Using Save
Depositing assets to Save
Withdrawing assets from Save