Arbitrum Orbit chain (Arbitrum chain)
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Applies to: Robinhood Chain and other Arbitrum chains
In more detail
Offchain Labs launched the framework in 2023 under the Orbit name; Arbitrum's documentation now defines an Arbitrum chain as "any chain that is built using the Arbitrum stack", deployable permissionlessly, in a Rollup or an AnyTrust form. Robinhood Chain describes itself as built on Arbitrum Dedicated Blockchains and, in its differences page, as an Arbitrum Nitro Layer-2.
Three consequences show up in RPC. First, a sequencer orders transactions before they reach the parent chain; on Robinhood Chain it orders first-come, first-served, and a public sequencer feed streams the ordered blocks. Second, finality is staged: a soft confirmation from the sequencer, then the batch posted to Ethereum, then Ethereum finality, while withdrawals through the canonical bridge wait a separate 7-day challenge period. Third, block context differs from Ethereum: block.number inside a contract returns an estimate of the parent chain's block number, the chain's own block number comes from the ArbSys precompile at 0x…64 (arbBlockNumber()), and JSON-RPC blocks carry the parent-chain estimate in an extra l1BlockNumber field.