What does CAR CLS CH stand for on a rental car receipt?

It may stand for "Car Class Change", which could refer to a downgrade or upgrade in the car class type.

There can be a number of reasons for this. If your reservation originally stated a particular car class (such as "standard") and at the time of rental, you choose a car in a different class (such as "full" or "luxury"), that adjustment will be reflected on your receipt.

Whether or not a car class change actually costs you money depends on what you negotiated at the time of rental. You should have been told clearly, up front, the cost differences and what the total cost would be to you. Sometimes an agent will upgrade your car class at no charge (this may happen if they do not have a car in the size class you originally reserved but have something higher. It may also happen as compensation for some inconvenience or error on the company's part, as a goodwill gesture).

If you believe you were incorrectly charged for a change in car class, contact your branch and/or the company's corporate offices. I have heard claims that certain companies "train" their agents to perform such upgrades without the customer's knowledge (and this increased fee shows up later on their bill), but I have never personally witnessed or heard of such a practice at the rental agency I work for (Enterprise Rent-a-Car). Such a practice would be illegal, and would be against the corporate policy and corporate culture of Enterprise as I know it.