ACR's is an abbreviation for ALUMINUM COPPER Radiator's. The most common place to find these are in air conditioners , either window type units or the A Coil mounted inside the duct of your furnace or outside condenser unit of your air condition system. It is typically a fair amount of work to extract this type radiators from an AC system but the $$$ are pretty decent in many cases. After the radiator is removed from the system it still needs all the magnetic metal to be removed from the radiator also any plastic or other non Aluminum or copper items. The Clean Aluminum and Copper can then be left together after cleaning and be sold as a Clean ACR.
Note: If you have this type of radiators out of a system already but don't have a way to remove the magnetic ends if it has some , you can always sell the almost clean ACR as an ACR(FE). You get a little less money this way but it is also less work. Ask your recycling center if you need more information about these type items.