My understanding of Royal Thai cuisine are that they’re favourite dishes which were being served to the Rama kings of the current Chakri dynasty. The term was first used by Bussaracum restaurant as their main chef had worked in the royal palaces previously, and the dishes served to the royal household - with a pronounced refinement in their preparation and presentation (e.g. small, very intricate vegetable/fruit carvings) were reproduced for a paying clientele outside, albeit at high prices.
Bussaracum was where I had my first taste of Chor Muang and Rhoom back in the 1980s:
I think Bussaracum sets the benchmark where Royal Thai cuisine is concerned, and its dishes were replicated by other restaurants like Erawan Tea Room (Bangkok), Erawan in Kuala Lumpur (no relations to its Thai counterpart), Thanying (Singapore), etc.
