elev; above — the level
above it, worked out from the elevations when unwritten, written
(above <level>, above none) when the next level up by elevation
isn’t the one; and height, worked out from above when unwritten
(the topmost level repeats the story below). A story height is a fact
about the building, stated once — a deliberate divergence from Revit,
where a level is only a plane and top-of-story is repeated on every
element. Elements with a base default to their level’s base and top (the
file’s level statement, or the unit’s one level); base <level> [± offset] (revit-parity: Base Constraint + Base Offset) and
height/top override per instance — top above reaching the
file’s next level up. A level has above; an element has top.
Placement: an elevation — elev 0'; every ẑ slot references the
level by its id.