Many of the solid design features allow you to include a draft angle, but they require that all faces be drafted the same amount. Draft a face allows you to set a draft angle on one or more faces individually.
To draft a face properly, the Fixed reference surface determines how the face is rotated. When a face is drafted, the surface must be rotated around a particular axis. This axis is determined by the intersection of the face and the fixed reference surface. By default a face is rotated around its intersection with the XY plane of the current UCS. This means that for a face that intersects the XY plane, a positive angle keeps the top of the face fixed and rotates the bottom of the face in:
The same result is calculated if a surface connected to the top edge of the face is selected as the fixed reference face.
If the surface connected to the bottom edge of the face is selected as the fixed reference, the bottom of the face stays fixed and the top edge of the face is moved out:
To draft a face: