For example in Tx, if your GWR or GCWR is over 26k, then you are under CDL...private, commercial whatever. If you are private not for hire you register that way, if for hire you register that way.
I believe farm tags limit is 54k in Tx, not sure on that yet as I have never been that route...will be doing that soon though.
You need to find out your home states regulations on weight regs and be prepaired.
If you are crossing state lines that involves USDOT regs, which the GWR or GCWR is 10,001 lbs for commercial or private carriers, so if you are hauling your tractor or whatever for a paying job..the regulations read"in furtherence of business".. just be prepared if a trooper pulls you over and does not believe "I'm going to uncle Daves place to help him out". Be prepared to prove in court. Just never know these days.
Best thing to do is read the Code Of Federal Regulations, do a search for CFR, find the Transportation Regs and read. That way you know the rules, not just hear say from a message board.
In my experience as a CDL driver and as a Supervisor of a small fleet of Commercial and non commercial trucks, having good equipment and having loads secured per the regulations( and yes there are regulations on how to tie down different loads) is 80 percent of the battle.
For example I have 1 F-650 with a 24' bed, its registered for a GWR of 25,900 lbs. Does not pull a trailer, does not go out of state. Is not required by Tx law to stop at weigh stations. This truck has been ran down by troopers at weigh stations on several occasions because it "looks" like a CDL truck after my drivers passed and open truck scale. Drivers given a hard time by the troopers, etc...anyways I have them stop at scales now and show the registration and they are waived on.