What is the difference between rūpa and Kāyā? In reading about the 4 Satipatthana or about the 5 Aggregates, it seems the first category of each (form) is sometimes called rūpa and sometimes called Kāyā. Are these just two words for the same concept or are there subtle differences? Thank you.
My understanding is the following:


Rupa = external and internal matter or form. Externally, rupa is the physical world. Internally, rupa includes the material body and the physical sense organs.
Kaya = the material body and the physical sense organs.