A few days ago I posted a question about repairing a burst 3/8" soft copper
pipe. Today I posted a question about some water filter housings I bought
at an auction.
Amazingly, the questions converged, because in one of the boxes was a bunch
of fittings; including several 3/8" compression unions!

There are plastic and brass rings to go on the tubing. I am guessing the
brass are for copper, and the plastic are for plastic. Is that correct?
There also little lipped brass tubes; they go in plastic tubing? Can I use
these with PEX?