The BeagleBone Black ships with the HDMI output enabled, using pins 20-46 on the P8 header. If you're not using HDMI and would like to free up those pins for use as shown in Appendix A, The BeagleBone Black Pinout, you can disable the HDMI output. First, mount the boot partition:

# mkdir /mnt/boot
# mount /dev/mmcblk0p1 /mnt/boot

Then, open the bootscript with the nano text editor:

# nano /mnt/boot/uEnv.txt

Add the following line to the end of the file:

optargs=quiet capemgr.disable_partno=BB-BONELT-HDMI,BB-BONELT-HDMIN

Press Ctrl + X then Y to save and close the file. That new line stops the capemgr driver from loading the HDMI, and HDMI audio overlays at boot, freeing up the pins for general use.

Finally, unmount the boot partition and reboot:

# umount /mnt/boot
# rmdir /mnt/boot
# reboot

The HDMI output will now be disabled, and you'll be able to use P8.20-46, as shown in Appendix A, The BeagleBone Black Pinout. To re-enable HDMI, repeat the steps and remove the line you added to uEnv.txt.