Table 2 of AMS-2431/8 seems clear to me - although there is an error in footnote (2)! A "field" is simply the contained region that the sample is poured into. For example AWCH62 can be poured into a 1" x 1" container giving a 1 square inch "field". If the shot was a uniform 0.0625" (1/16 inch) then there would be 16 x 16 particles IF they arranged themselves in a perfect square-packing manner. That gives 256 particles. This "field of 256" particles is then inspected microscopically. The process is to be repeated 9 times so that about 2300 particles are inspected (9 x 256 = 2304). The allowed 63 marginal particle maximum is about 3% of that number. The allowed 2 unacceptable particle maximum is about 0.1% of 2300 - not 1%.
