STAKE on xDai is staked by validators and delegators to secure the xDai chain with the POSDAO (Proof of Stake Decentralized Autonomous Organization) protocol. A UI embedded in BlockScout is used by participants to add, move, and withdraw STAKE and claim staking rewards.
Validator sets are selected on a weekly basis - called a staking epoch - with a maximum of 19 active validators per staking epoch. If there are more than 19 validator candidates, validator pools with higher amounts of STAKE (total STAKE from validator + all delegators) are more likely to be selected. However, a random value is also used to ensure fair selection to each staking epoch.
Each validator pool splits earned rewards equally from a staking epoch. Distribution within a pool is determined by the percentage of STAKE provided to the pool by a validator and their delegators. For more information, see details on POSDAO.