Summary: In your project's sketch scene, click "Edges" on the upper toolbar. Select by clicking your desired line label and sub-line label from the drawer on the right side of of the screen. Then apply the label by clicking the target lines of the diagram.
Labeling lines appropriately helps ensure your measurements are as accurate as possible. Since lines such as rakes, hips, valleys, and steps use the pitch value to calculate their length, mislabeling or leaving lines unlabeled can lead to seriously underestimating the size of a roof. Check out this overview of roof lines for more help.
Sub-line labels relate to the application of specific materials such as gutters, drip edge, ice & water shield, flashing, and ridge vent. Assigning these labels allows you to get material measurements quickly and pull directly into the estimating suite. These measurements are also visible under Category Measurements on p.4 in the Sketch Report in case you prefer to estimate outside of RoofSnap.
Watch the video below to see the process:
Note: A recent update now allows line labels to be applied as lines are drawn. Learn more here.