[Tagging] Tourism=attraction: feature or secondary tag?

[Yves]

tourism=attraction can be added to a lot of features indeed, that's why I think the label rendering in OSM-carto is a good idea because you will probably never find a common rendering to encompass this variety.

But on another topic, where does the idea of 'primary' and 'secondary' tags I read here and there and more and more often comes from? Are we making a map of the world in its complexity or what?
Yves 
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