The Face strategy automatically uses the extent of the setup stock as the machining area.
The selected stock
The default stock boundary
By default the facing is done in the Z level at the top of the model. However, this is easily changed by selecting Bottom Height on the Heights tab.
Setting the facing Z level by setting the Bottom Height to Selection and picking a point on the model
To set the machining area manually, simply click the Stock selections button on the Geometry tab and select the contours as usual.
Click the Stock selections button to manually select geometry