_I is a special class of link-types that is generated by the parser for idiomatic strings listed in the dictionary.
at_hand in_store: MVp- or Mp- or Pp-... +-Pp-+-_ITB-+ | | | Peace is at handNote that the _ITB link used here is not explicitly listed in the dictionary. Rather, the phrase "at_hand" is simply listed as an idiomatic string. In reading in the dictionary, the parser generates arbitrary link names, always beginning with the letters _I, to link the words of the idiom together.
Little can be said in general about idiomatic expressions. Probably the largest category are prepositional-phrase-like constructions which cannot be handled as ordinary prepositional phrases for some reason. In the case of "at hand", for example, the singular noun "hand" normally requires a determiner. Some idioms are more like adverbs, in that they cannot be complements of "be": "by_car", "in_turn". Some are actually like prepositions, modifying a noun or verb and taking another noun as an object: "next_to", "in_back_of". And some are like conjunctions, taking a clausal complement: "now_that", "in_case". The dictionary also includes a few idiomatic noun phrases ("head_of_state") and verbs ("do_so").
Grammar Documentation Page.