Facing is shaving the outer edges of the bottom bracket to make them
perfectly parallel before you install the bearing cups. Same goes for the
head tube before you install the headset. This kind of work is less
important now than in the past.


In what parallel universe? Just as important today as itever was.

Grinding material from the outer faces of the bottom bracket shell to make
them level and parallel*. Park Tools have more info on their website.

the material is not ground but milled.
Phil brown