Sequentia blocks are anchored to Bitcoin blocks for security and interoperability reasons; therefore, Sequentia nodes need to be connected with a (self-hosted) Bitcoin full node to run in a fully trustless fashion.
Each Sequentia block is anchored to a Bitcoin block for the sake of interoperability of smart contracts (e.g., hash timelock contracts, also knowns as HTLC):
Any new Sequentia block includes a reference to the hash of a Bitcoin block. If Sequentia block X references Bitcoin block Y, the block X+1 necessarily references the hash of a Bitcoin block at a height higher or equal to Y. When a Bitcoin chain reorganization happens, the Sequentia node discards all Sequentia blocks referencing the hashes of orphaned Bitcoin blocks.
Several Sequentia blocks can include the reference to the same Bitcoin block hash since the frequency of Sequentia block generation is higher than Bitcoin’s.
The anchoring allows faster and more reliable cross-chain operations between Bitcoin and Sequentia blockchains. Any operation involving two different chains, like an atomic swap, today requires waiting for long confirmation times (several blocks on both the blockchains involved) to ensure that the operation will not be reverted in one of the two chains. In this case, if Bitcoin reorganizes or orphans one or more blocks, Sequentia also reorganizes all blocks referencing the Bitcoin blocks discarded by the network. Therefore, if an operation is removed from the Bitcoin history (e.g., the atomic swap), the corresponding operation on Sequentia is also canceled from history.
It is important to note that anchoring does not allow for more reliable cross-chain transactions without other assumptions detailed in the following paragraphs (persistence §3.5, consistency §3.7). It will also be explained that the anchoring mechanism is exploited for other purposes relating to the properties of liveness and security of the blockchain (escaping stall §3.8, checkpoints §3.11).