A vehicle with a Gross Vehicle Weight Rating (whether or not a CDL is required is determined by the weight rating, not the actual laden weight) of 26,000 lbs. or less does not require a CDL unless:
It is part of a commercial use combination with a Gross Combination Weight Rating of 26,000 lbs. or more. So, for example, let's say you had a landscaping business and a Ford F650 (GVWR of 26,000 lbs.). By itself, you wouldn't need a CDL. But if you pulled, say, a trailer with a GVWR of 16,000 lbs. to haul a bobcat behind it, then you've created a combination which requires a Class A CDL.
It is hauling a quantity of hazardous materials which requires the display of placards. This would require a Class C CDL (minimum) with hazmat endorsement.
It is a bus designed to transport more than 15 persons (including the driver). That would require a Class C CDL (minimum) with passenger endorsement (or school bus endorsement if the vehicle is a school bus).