The tagset used by the TreeTagger is a refinement of this tagset: The second letter of the verb part-of-speech tags is used to distinguish between forms of the verb "to be" (B), the verb "to have" (H), and all the other verbs (V). So, "VHD" is the POS tag for the past tense form of the verb "to have", i.e. for the word "had".