Clicking one of the enum values in the XRStationaryReferenceSpaceSubtype IDL definition (https://immersive-web.github.io/webxr/#enumdef-xrstationaryreferencespacesubtype) goes to text in the XRStationaryReferenceSpace section.

More generally, all the content this section from "There are several subtypes ..." to the Note on position-disabled really applies to use in XRReferenceSpace or requestReferenceSpace().

Related to that, XRReferenceSpaceOptions and its enums are only actually used in XRSession::requestReferenceSpace(), so it's maybe a bit odd for them to be defined in XRReferenceSpace. XRReferenceSpaceOptions and XRReferenceSpaceType aren't used by XRReferenceSpace or any of its child interfaces, and XRStationaryReferenceSpaceSubtype shouldn't be (#513), so it probably doesn't make sense to define them there.