Facing is fabric applied to the inside of the garment to (i) finish off the raw edge, (ii) strengthen the seam, and (iii) help maintain the shape of the garment. Facing is mostly used on necklines, armholes and buttonstands. Facing is used to strengthen and finish off seams.
The top to the left is the actual garment.
The orange shading on the top to the right shows the facing if you could see through the garment.
The facing is on the neckline and button stand, to strengthen those seamlines. In this case there is no facing around the armhole because there are sleeves.
This top has facing on the neckline, armhole and sleeves. Note that in this case the sleeve facing is separate, but it can be designed so the sleeve, neckline and button stand facing is in one piece.
In this example, the neckline and armhole facing are done as one pattern piece.