[Tagging] Basic philosophy of OSM tagging

[Warin]

This comes from the tap discussion but has implications elsewhere.

What is the basic philosophy of OSM tagging at the top level?

Are 'we' tagging for

What things are? eg highways

OR

What things are used for? eg amenity

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Explanation? By example;

Highways are used for transport so would be better tagged as transport=motorway, sub tags for vehicles etc.

OR

amenity=drinking_water would be better tagged as water=blubber

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Is there an FAQ on this? Or has this never been documented/though of?
Have fun with this  :)