Adding air conditioning refrigerant

Which little dust cap do I remove in order to add air conditioning refrigerant? There are 2, one has a "L" on it & the other has an "H" on it. Each being a grey cap........Please help

ALWAYS the low pressure side (L). You can't hook it up to the high pressure side because the connectors are different. If you did, when you hooked the can up, it would prolly pop, or get REALLY hot.