First, thank you to Jim Goldfinch and Ed2345 for their attention and willingness to assist with this problem.
I'll start from the beginning. I would like to accomplish two things:
1) Create a HI-RES smart playlist that includes all (and only) songs in High Resolution Lossless Audio.
2) Create a LOW-RES smart playlist that includes everything EXCEPT the files included above (basically, all AAC and MPEG files).
Some background:
When in the Songs section of the Library, you can use the View>Show View Options command to display each song's attributes, including Kind. High Resolution Audio is listed here as HLS media, and other files are Purchased AAC Audio or MPEG, as shown below.


This would suggest that to get a HI-RES smart playlist playlist, you'd just have to say Kind contains HLS:

Here's where it gets weird. The above command returns a list of thirteen Doobie Brothers songs (yes, I'm old), all of which turn out to be Dolby Atmos rather than High Resolution.

Note that the image above the list shows album covers that are NOT Doobie Brothers. Makes me think there's something additional that's wrong with the output.
So, here a number of things I've tried, with their results:
Kind Does Not Contain AAC returns the Doobie list.
Bit Rate>320 megabytes returns nothing.
Size>25 megabytes returns the Doobie list.
Oddly enough, Kind Contains AAC returns the proper list, and when added to Kind Contains MPEG, provides me with everything that IS NOT High Resolution. In other words, my LOW-RES smart playlist.
There we have it--I think. All comments and suggestions welcome.