hi frends,
here is my point of view
choRu - cooked rice,
this could be justified from the proverb:
oru paanai ssotRukku oru ssoRu padham. ( to find whether a pot of rice has been cooked or not - a grain can show)
saadham - items mixed with ChaRu to form saucy/fluid nature (kuzhaivu) or food item made of choRu,
this could be justified from the menu items in the hotels, daily usage in home as saambaar saadham, rasam saadham, thayir saadham, lemon saadham.. etc
and obviously arisi is raw rice
hope this helps