An LNB needs a supply voltage. The level of this voltage switches the polarization. The AMSAT-DL DownConverter V3d has two 75-Ohm LNB connectors which are both connected to a dual LNB. For the NB transponder a voltage of 14V and for the WB transponder 18V is needed.
This voltage is generated directly on the board and coupled into the LNB connectors as phantom power. This supply voltage is short-circuit proof and is monitored by software. If you want to install the LNB rotated by 90 degrees, you can use two jumpers to rotate the polarization direction correctly again.
14A is for the NB LNB (position shown in the picture: 14V)\\. 14B is for the WB LNB (position shown in the picture: 18V).