What it is.
OptionallengthFence segment length, metres (fences only; default 2.5).
OptionallinePower line id: poles and pylons with the same id are strung together, in placement order.
Per-instance seed: size, colour and detail variety.
OptionalvariantOne of the kind's PROP_KINDS variants (default: the first).
Position (frame-local), metres.
Ground height (local y), metres.
Rotation about up, radians (0: front faces +z).
Position (frame-local), metres.
One prop instance. Plain JSON. Local axes: +y up, +z front (towards the road), fences run along x.