The hymenium is located beneath the cap and contains the spore-forming organs of the fruiting body, i.e. spore sacs (asci) or microstructures (basidia).
Sources
Britannica – Hymenium 
Natur & Kultur – Unusual Edible Mushrooms by Oliver Karlöf
April 2023, TÄNKOM | Revised February 2024 RETHINK
