When a user shorts any moAsset, the user is minting the moAsset against their collateral and short-selling it automatically in the same transaction. The system records the debt in the moAsset shorted Unit.
After opening a short position, the user ends up with the equivalent value of the shorted position quoted in moUSD. In order to close their short position and redeem the collateral, users are required to pay back their debt in the moAsset that was shorted.
For instance, User A shorts 1 moBTC by staking DAI. Mobius Finance platform will mint 1 moBTC and sells it automatically for moUSD. User A receives moUSD equivalent to the value of 1 moBTC. The user debt is represented as 1 moBTC and the user will need to pay back moBTC as the debt in order to get their collateral back.
Let's suppose that the value of moBTC drops by 10%. User A uses 90% of the moUSD to buy back 1 moBTC and pay back the debt. This allows user A to redeem its original staked asset, and have a return of 10% of the moUSD minted by the protocol at the start. The key point to understand is that user A debt is represented as moBTC, while other users who mint moUSD and buy moBTC have their debt represented as moUSD.