The Capital “M” is stand for Mega (10^6), the small case “m” is stands for mili (10^-3), the "r" is representing the reactive power (j*V*A), which is quadrature to the real (active) power.

When AC power is calculated the real power “P” is in W, kW, and MW. The reactive power “Q” is VAr, kVAr or MVAr. The vector sum of the real and reactive power is called apparent power “S” and its units are, VA, kVA, and MVA.

Best regards,

Joe