Layer	Type	How BTC / assets move	Security / trust at bridge	Programming model	Lightning interop	USDT today
RGB	Client-side validation on L1	Off-chain consignments; Taproot commitments (Tapret).	No indexers required for correctness (you must keep proofs).	Client-side contracts (AluVM).	Emerging via Bifrost / RGB-over-LN.	Announced Aug 28, 2025; not generally live to end users yet. (Tether)
Taproot Assets (TA)	Asset protocol on L1/LN	Taproot commits + off-chain proofs; optional Universes.	Trustless validation; Universes aid discovery/restore.	Asset-focused (issuance/transfer/audit).	Shipping over Lightning since 2024. (Builder's Guide)	Announced Jan 30, 2025; many wallets show wrapped until Tether mints. (Lightning Engineering)
Spark	Statechains-based L2	Off-chain key handoffs; pre-signed unilateral exit with decrementing timelocks.	“t-of-n” operator trust at transfer; unilateral exits as safety valve.	Token/API layer (BTKN).	Via service providers (BOLT11 flows).	No Tether-issued USDT.
Liquid	Federated sidechain (Elements)	Two-way peg; 1-min blocks; Confidential Transactions.	Strong Federation of functionaries.	Simplicity smart contracts live on mainnet (Jul 31, 2025). (Blockstream)	Parallel to LN; broad wallet/CEX support.	Live USDT for years (officially supported network). (Tether)
Rootstock (RSK)	Sidechain (EVM), merged-mined	PowPeg 2-way peg; BTC↔RBTC.	Federated HSM signers (PowPeg) + miner security.	Full EVM.	Not native; via bridges/wrappers.	USDT0 live (LayerZero OFT deployment).
Citrea	Bitcoin ZK rollup (EVM-equivalent)	Sequencer posts commitments (state root + diffs) to Bitcoin; zk proofs posted to Bitcoin; full nodes settle by verifying commitments against local execution. (Citrea)	Bridge (Clementine) is BitVM-based and runs a Bitcoin light client for trust-minimized two-way peg; separate from rollup settlement. (Citrea · Blog)	EVM equivalence (zkEVM). (Citrea)	Needs connectors/bridges; not native LN.	No public Tether issuance.