Ellipsis has two types of pools:
Base pools: 3Pool (BUSD-USDC-USDT), btcEPS (BTCB/renBTC)
Metapools: for example [fUSDT, [3EPS]], [DAI, [3EPS]] or [TUSD, [3EPS]]
Metapools allow for one token to trade with another underlying Base pool. In this example users could seamlessly trade fUSDT between the three coins in the 3Pool (BUSD-USDC-USDT). This is helpful in multiple ways, including:
preventing dilution of existing pools,
allowing Elipsis to list less liquid assets,
providing more volume and more trading fees for liquidity providers.
A Metapool consisting of [fUSDT, [3EPS]] would contain fUSDT and 3Pool LP tokens (3EPS). This means that liquidity providers of the 3Pool who do not provide liquidity in the [fUSDT, [3EPS]] Metapool are shielded from systemic risks from the Metapool.