The engine axis of the ADR-009 validation context: the sitemaps.org baseline
plus the opt-in google, bing, and yandex overlays (sitemap-spec §12.0).
sitemapr owns this value set and it is extensible; this accessor is the one
canonical source vector every ruleset-aware surface validates against. The
baseline "sitemaps.org" is deliberately first so it is the default choice.
See also
ruleset_revision() for the published revision of a ruleset and
ruleset_context() for the per-source context axes.