Normally, scaling is done in all directions within the profile plane and with an attachment point (An intersection of a profile's plane and a spine) on the spine as the center. With the Bidirectional Scaling option, you can specify two scaling functions which have their influence in two specific directions orthogonal to one another.

When Bidirectional Scaling is selected, two vectors are shown at the start and end of the spine in the "up" direction. In the Guided Loft dialog, the vectors are defined in Start Up Dir and End Up Dir. The two vectors are interpolated along the spine and can be influenced by using the Orientation Options.

Modify these "up" vectors by specifying an option for Up Scale and Side Scale. The Side Scale function has an orthogonal influence on the Up Scale option.