USDOT Number Decals are used on vehicles which are transporting goods across state lines. These are Federally Regulated Licenses.
If your vehicle carries a US DOT number, and is based in California, it also requires a Motor Carrier Permit (CA number) issued by the state. (See CA Numbers section for that information).
You must ALSO include the Company Name lettering with the company listed on the truck registration. The name must also be clearly visible, but can be created with fonts having more design to them, such as Times New Roman or the infamous Comic Sans. If your name is difficult to read, you could get a traffic citation.
If you are driving a TRACTOR CAB, that information needs to be clearly visible on each side. You can place the information on the door or on the sleeper sides. If you are driving a BOX/STRAIGHT truck, you would need to have the identifying numbers on the cab doors or on the front lower corners of the box, and the name can be either above the numbers on the cab doors, or on the sides of the box only.
If you are hauling a trailer, you must have your company registered name vinyl decals lettering and the USDOT numbers decals on the tractor, regardless of what is on the trailer.