I think I found the info you're looking for on the FMCSA site.
Question 2: Is a driver of a combination vehicle with a Gross Combination Weight Rating (GCWR) of less than 26,001 pounds required to obtain a CDL even if the trailer Gross Vehicle Weight Rating (GVWR) is more than 10,000 pounds?
Guidance: No, because the Gross Combination Weight Rating (GCWR) is less than 26,001 pounds. The driver would need a CDL if the vehicle is transporting HM requiring the vehicle to be placarded or if it is designed to transport 16 or more persons.
Regulations Section | Federal Motor Carrier Safety Administration
Thank you, although I checked into CDL at one point, and the C being short for Commercial apparently relates specifically to whether the vehicle is being used for Commercial use. This may explain why I see "not for hire" on some larger vehicles so they clearly denote they are only used privately.