I'm finding it confusing how to search for models that have "USB serial" for application use. Is there a better term for it?
I will be making a PC based UI that will communicate over serial with the Arduino, to send some simple commands and read some values (DIO/AIO), updating the UI. I expect that I can open a COM or (/dev/ttyS0) and write packets to it over USB-Serial (CDC) class (and not a separate UART)
Which boards support this?
In reality any board supports serial I/O.
Your sketch will determine how the serial is bot set up speed / baud wise and the data that is passed to or from the PC.
Most boards support a single UART approach or even a software serial method.
Some larger boards such as the MEGA have more UART availavle.
I'm sorry, I don't mean to be dumb, but why would USB-serial care about the baud? It's over the USB interface.
I don't care about any hardware or serial UARTS.
I'm talking about Arduinos that already have that built in. (I think that's the right statement?)
Sorry I thought that was the crux of your Q.
I may be confused. And I appreciate your help.
In some Arduinos, the "USB serial" communications pass through two interfaces, the USB interface via an adapter chip, and the standard UART serial interface. The latter has a defined Baud rate.
Oh, so which ones have the USB interface? I'm under the impression that's not the normal way?
There are two different kinds of USB interface, native USB and UART with USB adapter (the Arduino Pro Mini has neither). Check the specs of any Arduino that interests you.
What key words am I looking for if I am only interested in "native USB serial"?
Your guess is as good as mine. Some of the ARM-based Arduinos have native USB interfaces. Teensy, for example.
Do your research.
That was my impression as well, but I've searched, it isn't really a great guide like there is for the Teensy: Teensy Technical Specs Comparision Table
The website is like each board is an island, and offers no easy way to compare.
ATmega32u4 Arduinos, like the Leonardo, also have a native USB interface, and it does not matter what baud rate one sets, but you will find that discussions on this and other forums about the actual data transfer rate are confusing.
I believe that the boards listed as able to use Circuit Python have the native USB in the sense you are using it. Here's the listing of Arduino, Adafruit, and Teensy boards. You can certainly search for others, but I'm not certain about which ones can use the Arduino IDE. I believe that the Leonardo is not on the list because of the 16 bit architecture.
https://circuitpython.org/downloads?manufacturers=Arduino&manufacturers=Adafruit&manufacturers=PJRC
Here are the official boards under active production with "native USB" capabilities:
As mentioned already, there are quite a few other 3rd party board options as well. The only ones of those I have experience with are the Teensy boards and the SparkFun Pro Micro, which are very nice.
Interestingly, here: Adafruit ItsyBitsy 32u4 - 5V 16MHz : ID 3677 : $9.95 : Adafruit Industries, Unique & fun DIY electronics and kits It says it is the same as the leo, but has USB 'CDC' serial device. So maybe the leo can?
PS. Rereading, You probably meant CircuitPython, not USB-CDC.
That's what the "U" in the device name is telling you. 
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