This site describes solve-engine as it is on main: 2.43.0, which npm does not have yet. npm installs 2.40.0, so a page may show an answer that version does not give yet.
UNCERTAINTY_PACKAGE
const UNCERTAINTY_PACKAGE: IEnginePackage;Defined in: packages/engine/src/packages/uncertainty/UncertaintyPackage.ts:16
Uncertainty propagation: a measurement that carries a one-sigma tolerance,
written a ± b or the ASCII a +/- b, propagated through +, -, *, /
for independent errors combined in quadrature.
The ± symbol is lexed directly; the ASCII spelling is fused by the
normalizer rule. Both reach the one infix parselet, which emits
MAKE_UNCERTAIN. Correlated errors are out of scope (a much larger problem);
a comparison or transcendental function reads the center and drops the
tolerance. See vm/VMConversion.ts’s uncertainOp for the propagation rules.