The "Welsh Code Profile" indicates the extended or total coding method rather than the original (MMPI only) coding method.

For all scales noted, moderate scores are generally considered normative. You should note that everything is open to interpretation. There is no simple one-to-one this-means-that interpretation for the MMPI2. You have to examine the results as a whole.

L is the "Lie" scale. Very high scores on L indicate Faking Good (trying to appear more 'normal' than you are, resistant/naive) Very low scores on L indicate Faking bad (trying to appear worse than you are, sarcastic/cynical)

F is the "Infrequency" scale. Infrequency means that these items are rarely, rarely endorsed. Very High scores indicate an invalid test response; either not understanding, resisting the test or again, faking bad. Low scores are considered (more) normal

K is the "Correction" scale. Its used to measure openness/defensiveness. High "K" scores could indicate serious defensiveness, shyness, faking good or lacking insight. Low "K" scores indicate either invalid answer patterns (faking bad or all trues,) poor self-concept, cynicism, or or a (self/other) critical nature.

The formula you mentioned: L'+F-K is firstly because you can choose to score the MMPI2 as either K corrected or uncorrected. Not all scales are K corrected. The reason you "K correct" is to adjust for test taking approach. Your socioeconomic status usually influences your K status, btw!

I can't say much more than this