The vocabulary containing words  forms is updated as more training examples are seen and used by the feature builder.
given enough training samples of the sentence Q, the variable W and constraint R, we can deduce the pattern , which identifies how R constrains W. This pattern contains information about the ordering in Q with respect to W (recall W could be vector-valued) such that they are mapped to R. to say, every form has some specific interaction pattern with other forms when they exist together/adjacent in Q.