This is all I can find.
The BC means the Back Course hold and it is for runways that have ILS on only one of the sides....example rwy 34 has ILS but 16 doesn't. Well if you would just enter the 160 to be the course it wouldn't enough....not for the AP because then the needle would be mirored...and that would make bannanas with autopilot. So you are too left and the neddle shows you are to right, the ap goes left because the AP only sees where the needle goes. So it would be an endless attempt to go right by turning left. The BC only fixes that problem...so that when approaching fromt he other direction you can use the ILS at least for lateral navigation.
The Localizer Back Course
"...the localizer antenna array is located at the far end of the runway, away from the approaching aircraft. Its antennas are arranged and manufactured in such a way to transmit a narrow signal on the runway approach path. That localizer signal is so accurate that with its guidance, along with the glide slope signal, an aircraft can safely land when the visibility is zero and when the ceiling is zero.
However, the Localizer antenna array also radiates a narrow signal beam in the opposite direction of the approach path. That signal is called the Localizer Back Course."
I took this as is, not my words.