Alongside, religion, many religious things such as amenity=place_of_worship have a denomination, that distinguishes between different sects of a religion. There’s a list in OSM at taginfo. One struck me as odd - denomination=landmark, since that doesn’t seem to be a denomination of a religion. I’m mentioning this here because it looks from the map that many are mapped in India.
Is it perhaps a typing error for denotation? That’s often used to indicate things to navigate by (mostly trees, for historical OSM reasons**)?
** Initially natural=tree was interpreted by some as “only for significant trees”, but as people started mapping more trees including “insignificant” ones, the denotation tag was invented.