Milliampere or "milli", sometimes called "macro" signifies that the amount of electrical current being supplied by the device is of the milliampere level.  Milli refers to 1/1000 or .001, so one milliampere is .001 ampere, a small amount of elecricity.  An ordinary light bulb uses 1.0 amperes of electricity.  In normal usage, the machine supplies under about 10 milliamps or less to the acupuncture needles. 

Microampere or "micro" refers to electrical current supplied at the microampere level, or 1/1,000,000 of an ampere, or .000001 ampere, a very small amount.   

The cells and tissues of physiology respond to  current levels differently, and some tissues respond to micro current best, and some respond to milliampere current best.  This can be used to specify treatments.