Part Modeling > Base Features > Sweeps > Sketch Plane Orientation > Normal to Trajectory (Automatic): Start X Direction
  
Normal to Trajectory (Automatic): Start X Direction
The z-axis is tangent at all points to the origin trajectory. You can select the x-direction at the start to sketch the section in a preferred orientation. For some trajectories, for example, some straight lines (or non-sketched lines), there is no intrinsic default orientation. Therefore, you must select an x-direction at the start. The system cannot automatically set the initial x-axis position.
The xy-direction is determined by the projection of the direction defined by the reference along the z-axis.
1. Straight datum curve selected as an x-direction reference and projected on the sketch plane along the z-axis