The THF folder contains the skeleton of an instance of Logic.Logic for THF, a variant of TPTP for higher-order logic, see http://page.mi.fu-berlin.de/cbenzmueller/papers/J22.pdf. Note that in the meantime, TPTP itself also includes higher-order logic. Hence, THF is superseded by TPTP now. Still, THF may contain some functionality that is not yet provided by TPTP.