Rules for Creating a Datum Curve with the On Surface Option
Consider the following rules for a curve created through points:
• Selecting 
Place Curve on Surface might conflict with some of the previous settings that you defined for that curve. For example, the points selected to define the curve lie on the surface, and the tangency conditions must be possible for the selected surface. When a conflict is detected, you are prompted to resolve it by removing invalid references.
Place Curve on Surface might conflict with some of the previous settings that you defined for that curve. For example, the points selected to define the curve lie on the surface, and the tangency conditions must be possible for the selected surface. When a conflict is detected, you are prompted to resolve it by removing invalid references.• When you select Place Curve on Surface, you cannot select the Tweak curve check box on the Options tab.
Place Curve on Surface, you cannot select the Tweak curve check box on the Options tab.