Custom Seam Allowance... (Additions Menu)
Modes: S U

This is a feature that allows the user to select a segment and set a custom width for the seam allowance on that segment. For example, you may want to have all curved seams have 1/4 inch seam allowance and all straight seams to have 1/2 inch seam allowance. To set a custom seam allowance, on the Additions menu, click Custom Seam Allowance. When the dialog opens, type in your custom width (and name if you like). Click OK and the window will close and your custom seam allowance will appear.
 
 
Hot Tip:
How do I quickly add a mix of 1/2 and 1/4 inch seam allowances?
You can do this relatively quickly by setting a Global Seam Allowance of 1/2 inch on all pieces. Then, you Shift-Click-Select all the curved lines, choose the Additions/Custom Seam allowance option and type in the custom measurement of 1/4 inch. Voila!