This is a facing used for slits in a garment. The Slit Length will be set automatically if this is part of a garment, otherwise you will need to set it manually.
The width of the facing is set with the Width Amount variable.
The Facing Cut Number is the "cut" value that will be printed on the pattern. The Seam Allowance is applied only to the bottom of the pattern.
The height of the facing is the Slit Length plus 1.5" / 38mm.