The HPF should be a high pass filter. Uses, if you are recording something that has no usable information in the lower register you would omit this part of the frequency spectrum using a shelved eq from whatever point you choose and thus would let the higher frequencies pass. hence high pass filter! Mostly used for keeping rumble and handling noise out of the scource. The phase reverse does just that, reverses the phase. Uses, 2 mics picking up the same scource at different distances may be subject to phase cancellation as one waveform reaches the first mic at a diferent part of the cycle that the second mic recieves and when put together there is phase cancellation. the second mic can then be phase reversed to compensate for this. But by no means is that a panacea for incorrect mic placement to begin with but is only a quick fix.