Underlay

Definition:
The Underlay is the foundation of the design. The Underlay helps stabilize
the material and helps the threads lay properly on the garment. In many
cases, when you need to increase stitch coverage of a shape, you can add
more underlay versus adding more density to your shape. This helps provide
added coverage while minimizing the risk of puckering and high stitch
counts.

Some form of underlay is present on almost all digitized shapes. You can
use a single underlay type or in some instances you may want to use multiple
underlay types.