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.
VARIABLES_PACKAGE
const VARIABLES_PACKAGE: IEnginePackage;Defined in: packages/engine/src/packages/variables/VariablesPackage.ts:16
Variable read and write WITHIN one document: :name = expr to define,
name to read it back.
Document-spanning variables are a separate package,
solve-global-variables, because they are a different level of
functionality: everything here stays inside the document being evaluated,
where global :name reaches outside it. Keeping them apart is what lets a
host refuse one without losing the other. See that package for the
reasoning.