xMATE is a token you receive in exchange for staking MATE tokens in the single asset vault (SAV).
To receive xMATE, MATE holders have to stake their MATE in the SAV to receive xMATE plus additional MATE, which derives from the open market with the 25% of funds collected from the platform's trading fee.
For every limit order executed on Mate's platform, 0.2% is charged on the traders, and 0.05% will be used to buy back MATE tokens and add them to xMate pool. The amount of xMATE reward is proportional to your amount of MATE staked in the SAV.
When you stake your MATE tokens, you "purchase" a share of the xMATE pool. Because xMATE appreciates in value compared to MATE, the amount of xMATE you get for your MATE decreases over time.