What is the difference between an Alignment and a Linear Feature Definition?
Alignment Feature Definitions have the Corridor Template Property.
This allows an auto placement of a corridor when Use Feature Definition Template is toggled on. It also works in conjunction with Create 3D Automatically. These two features will place a horizontal line that would make a vertical and create a corridor all in one click.
Linear Feature Definitions have the Create Template Geometry Property. This creates 2D geometry from corridor linears so you can place a Civil Cell on it. Typically, any Feature Definition used in a Template will have this set to true.
They can be used interchangeably, unless you want to utilize one of the features listed above.