when you first boot up with a freash sd card in a bbb, it loads up, (then asks to expand, instead of expanding, you go install to emmc)Only if you choose to copy it there after doing the install, which it is warned against doing where the button is to do it.
If you build the SD card on a PC etc, and drop it into the BBB, it'll run from the SD card until told otherwise.
then the sd card is only for data