The two STEP template panels—STEP Template Content and STEP Template Frame—are used primarily to create product templates in InDesign. These panels (also referred to as panels in various locations throughout this guide) are where elements from STEP—including attributes, asset references, and tables—are linked to InDesign pages to build mounted pages in conjunction with publication templates. The STEP template panels can also be used to link these same elements to page templates and publication templates, though this is less common.
The two template panels are detailed in the following sections of this documentation—STEP Template Content (here) and STEP Template Frame (here).