A Lexicon tells you which tags are allowed at a particular point in a sentence.
A simple lexicon that thresholds to decide when to open up the rare word to all (open) tags
A simple lexicon that thresholds to decide when to open up the rare word to all (open) tags
A fairly dumb tag scorer that does some simple smoothing to estimate p(t|w).
A TagScorer is just something that assigns scores to a particular POS tag (or whatever) at a particular position in a sentence.
A simple lexicon that does no smoothing at all.