To which airspace does the border of two adjacent airspaces belong to?
You recall correctly. That document is ICAO SARPs Annex 11 § 2.6:


  Where the ATS airspaces adjoin vertically, i.e. one above the other, flights at a common level would comply with requirements of, and be given services applicable to, the less restrictive class of airspace. In applying these criteria, Class B airspace is therefore considered less restrictive than Class A airspace; Class C airspace less restrictive than Class B airspace, etc.


(Emphasis mine.)