Ah, Larry, you may be right. Sometimes Apple's extreme economy with language makes it hard to grok what is happening for those of us who understand via examples. Here is my situation:
The photos on my SOURCE computer are RAW. I share them via photostream with an account on my TARGET computer. When they arrive on my TARGET computer, they are in TIFF. I presume this is because my TARGET computer is working from a shared photostream.
If that is the case, then it seems to mean that in order to edit the photos on my TARGET computer, I need to transfer them from my SOURCE computer to the TARGET computer by some other means (USB stick or airpdrop).
I think that solves it - thank you, and sorry for my density.