Embroidery Designs can comprise one or more EMB templates; they reside in Design Categories which are displayed as the first step in the front-end.
A typical example where you may want an embroidery design to comprise two EMB templates would be a Name. For example, you may want offer a single line first name option, and a two line option with first and second name.
The front-end handles Embroidery Design's comprising multiple EMB templates by displaying an entry-point thumbnail for the Embroidery design (this can be anything you choose) which drills down to the EMB templates.
You can see an example of the front-end on the left where an embroidery design thumbnail drills down to two options: a single name and two names - each with their own thumbnail.