"M" is light weight, thin-walled copper pipe normally only used in internal residential use.
"L" is medium weight, thicker-walled pipe that normally can be used in residential and commercial.
"K" is heavy duty, thick-walled copper pipe used for underground installation such as a main supply line from the meter.
The exterior dimensions of the copper pipe are the same for each type so that common fittings will make up.
Check with your Building Inspection Department to see what is required or allowed by local code in which application.
Good Luck!
Mike