A truss layout family consists of lines that define truss elements such as chords and webs. Chord and web members are created such that their center lines (local x axis) will lie along the layout lines that you define in the truss layout family. The entire layout will be transformed such that the distance between the 2 end reference planes will be determined by the truss instance based on its shape in the project. The Length parameter may be used in your truss layout family to perform calculations to specify the exact location of vertical web members or to calculate the number of panels to create in the project environment.
Truss family editor tools
Detail panel
Top Chord to draw the location and geometric configuration of the top chord layout lines. 
Detail panel
Bottom Chord to draw the location and geometric configuration of the bottom chord layout lines. 
Detail panel
Web to draw the location of vertical and diagonal web layout lines. 
Properties panel
Family Type to select the types of structural framing families used for chords and webs. You may create different types of the same layout family. Differences between these types include the following.