sdd:order
sdd:order materializes a sdd:Section's 1-based declaration position within its container. Unlike every other scene quad, which the canonical serialization reorders freely, sdd:order is exempt: it is data the flex pass reads to decide which fixed sections reserve space first, so its literal ordinal must survive round-trip unchanged (design spec §2).
A container's sdd:flow axis realizes this order visually — declaration order is visual order, with no separate layout step to reconcile the two.