Wordnet Lemmatization
The exception list files are used to help the processor find base forms from 'irregular inflections' according to the man page.  They mean that some words, when plural or a different tense, can't be algorithmically processed to find the base/root word.  More details can be found in the morphy man.  I'm not a language processing expert, but this is likely a result of English words that 'break the rules'.  If you think about the code like a human trying to learn English: the student learns rules to use (algorithm) and then has to memorize exceptions for the rules (exception lists).  An over-simplified analogy that does not involve endings/conjugation would a spell checking program.  An algorithm might check for 'i before e, except after c' but would first have to check the word against an exception list to make sure it isn't 'weird' or 'caffeine' - please don't start a linguistics fight about this rule, I am not commenting on the validity of it/that's not the point I'd like to make.