Each template has a @group that indicates where the template fits in an application page.

The group restriction serves two purposes. For top-level container templates, such as those that define a page, a group restriction can be specified for each section of the page. This limits the cartridges that can be inserted into that section. For example, if a template that includes a "SecondaryContent" section, only cartridges of the "SecondaryContent" group are available to insert into that section in Experience Manager.

Additionally, you can specify a template group in a dynamic slot to restrict the content that appears in that slot. This restriction applies at runtime when content items are evaluated against each other and ranked by priority for display in the application; any content items that do not match the specified template group for a dynamic slot are removed from consideration.

"ecr:type": "template",
    "@group": "SecondaryContent",
    "@description": "${template.description}",
    "@thumbnailUrl": "thumbnail.png",