Facedir is a value that defines direction in which node's top face is directed and rotation over this direction.
Values range: 0-23
Usage:
Facedir/4 -> Direction
= 0 Y+
= 1 Z+
= 2 Z-
= 3 X+
= 4 X-
= 5 Y-

Facedir modulo 4 = rotation in the direction

Note: Facedir/4=0 and rest 0-3 is just old facedir.
Any old code not aware of new possible values will work as before.

About name. It fits, just now its not the front of the node just its top.
And direction (dir) means all possible directions.