The authoring mode is different from the free generation mode in that it gives the author the responsibility of choosing expansions for nonterminals rather than enumerating all possible expansions nondeterministically.
after all the obligatory expansions from dfa(D,F,A) (expansions for which there is only one possibility in the grammar) have been done, the frontier of the derivation tree contains some terminals and the nonterminals drug(D), form(F), admin(A), comments(D,A), and has to satisfy the constraint df(D,F).