sdd:weight
sdd:weight is a sdd:Section's proportional share of its container's remaining main-axis length. The flex pass reserves fixed sections first — a percent of the container's main-axis length, or an absolute sdd:length — in sdd:order, and only what is left after every fixed reservation splits among weighted sections in proportion to their sdd:weight (design spec §3).
If fixed sections over-commit, they clip rather than rescale, and weighted sections receive nothing; if fixed sections leave space with no weighted sections to claim it, the layout leaves it empty rather than stretching. Cross-axis extent is unaffected by weight — it is always the full container extent.