Rather than labeling 'Y' or 'N' for every character in standard words, they give each character the following 4 types of label: (1) one of the 0-9 digits; (2) one of the 26 characters; (3) null character '_', and (4) letter combination (they used 5 common combinations, such as 'wh', 'ck', and 'ey').
this system can handle other types of non-standard tokens, such as substitution.