XChain draws a hard line between READ (no state change) and WRITE (state change). Most cross-chain needs are just verifying that "X happened on chain A" — a READ, over Interchain Queries (ICQ) — which is cheap and effectively unlimited. Only atomic state change is a WRITE, over IBC v2/Eureka + Interchain Accounts (ICA), and it is deliberately kept scarce.

Type	Mechanism	Cost	Budget
READ	ICQ — verify a proof against an existing root	Cheap (one verify)	Effectively unlimited
WRITE within a Cell	Shared sequencer (atomic inclusion)	Low	Most stateful traffic
WRITE across Zone/Region	XMP + ZK light client + pessimistic proof	High, async	< 0.1%
Global atomic WRITE	T0 only; intent/solver underwrite	Very high	< 0.01%

The reference data path — recording a "like" and proving it cross-chain without re-execution:

Diagram

Figure 3 — The reference data path (MVC-1): append, anchor, and prove inclusion cross-chain.

Takeaway: a separate chain proves the "like" happened with a single proof check — no stateful message, no re-execution. Cross-region atomic WRITEs are not promised to be all-or-nothing at the protocol layer; they achieve containment through pessimistic proofs and economic underwriting.