I am trying to find a picture reference for you, but in the meantime in you HVAC system the metal lines in the engine compartment will have a Schrader valve service port. The larger in diameter size line is the low side. That is the side of system to add refrigerant and usually has a Blue plastic protective cap. If you are looking to add refrigerant as a DIY the cans sold that you buy can only be attached to the low side because the Schrader valve bungs are different in size between the High and Low side for that reason. I have attached a tutorial for you and hope this helps. I will continue to look for a picture if it helps as well.