Hi,
A Geosite (also Geoheritage https://en.wikipedia.org/wiki/Geoheritage) are places  of geological significance usually inside Geoparks (https://www.unesco.org/en/iggp/geoparks/about).

In Portugal there are 6 parks (https://www.visitportugal.com/en/node/426541) comprising several geosite(s).

These are places selected for their high geological relevance, that's why they all have the geological=* tag (except when they are viewpoints from where geological features, geosite, can be observed on the horizon).

On the ground they are used to attract tourism and educate visitors.

At the moment I'm not certain that this is the best tag to classify an officially designate geosite. But it will have to do for the meantime.

If you have ideias and want to contribute to a better tagging, please do.

RB

PS. C.f. "In the north, in Arouca Geopark you can discover 41 geosites with unique characteristics, of which the Pedras Parideiras de Castanheira (Castanheira’s Breeding Stones) or the Trilobites Gigantes de Canelas (Canelas' Giant Trilobites) stand out..." in https://www.visitportugal.com/en/node/426541