When a flare's intensity is determined, it is classified using a letter, followed by a number, which tells us the specific intensity of the flare. X-ray flare intensity is measured in units of power per area or Watts per meters squared. Each letter (A, B, C, M or X) represents a certain numeric value and the numbers following the letter in the flare classification multiply that value. The numeric values of the letter classes are:
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A = 1.0x10E-8 (W m-2)
B = 1.0x10E-7 (W m-2)
C = 1.0x10E-6 (W m-2)
M = 1.0x10E-5 (W m-2)
X = 1.0x10E-4 (W m-2)
(the W m-2 means Watts per square meter)
To determine the exact intensity of the flare you multiply the number in the x-ray classification of that flare by the value of its class listed above. For example, the X28 flare had an intensity of at least 28.0x10E-4 Watts per square meter.
This X28 flare is the most intense on record. We began accurate records during the 1970's. So, there might have (and probably have been) more intense flares in the past, but this one is the new record holder since the 1970's.
What all does that mean? Well, it was an event that caused complete shortwave radio blackout on the sunlit side of the Earth, and degraded somewhat the propagation of shortwave radio signals on the darkside of the Earth. Thankfully, this one was not directed straight at the Earth, but off away from the side of the visual sun.