they have different hardnesses. Mis softest and k is hardest. In massachusetts we use L for all water piping in the building and type K for the water service coming in from the street. M is used for heating unless otherwise specified.
is availabel (ASTM B 88) in K,L,M,DWV,Medical Gas,ACR and G. Wall thickness varies from .025"-3/8"M to .405"-12"K. There are other types available from the mill on special order. The letter designation has nothing to do with hardness. I have no idea where the designations came from. The first documented use of copper tube to convey water was 5,000 years ago. It was used to supply water to Pharaoh Cheops bath. It is still in usable condition. I wonder what 5,000 year old PEX will look like. bob
According to www.copper.org the K-L-M designation refers to wall thickness:
"Types K (heaviest), L (standard), and M (lightest) wall thickness schedules as classified by ASTM B88"
Am still searching for the origin of those designations. I've found that the origins of nomenclature are EXTREMELY difficult to find on the web and almost never mentioned in printed trade references. You have to find the "weird facts" sort of sources...