The intra-chunk ordering can be done in multiple ways.
The easiest one is ordering by fees (tip), and break ties as @edfelten suggested using receive-order. This adds negligible overhead for sequencers and will be good for settings where user preferences in burst periods are mostly common value (i.e., preferences are not diverse), which is the case in atomic MEV arbs or liquidations.
It is also possible to add more UX features into intra-chunk ordering. For example, the validators could add revert protection for users which reduces spam. Or, they could provide conditional payments like coinbase.transfer in Ethereum that allow users to express more diverse preferences which enable features that are already being explored by application teams building on Arbitrum (e.g., Shell Protocol’s design of a composable transaction/trade batching engine). If those additional UX features are wanted, the sequencers can implement themselves to have the architecture embedded within the sequencing node (with an estimated added end-to-end latency of ~6ms, assuming same transaction load as current Ethereum). If the sequencers don’t want to implement themselves, they can also work with third-party trusted services.