Templates become a set when they are grouped together. Creating Template Sets allows you to push the same data (images, texts) to generate multiple images via a single API call. The resulting images are grouped into an Image Collection.
Template sets allow you to push the same data across multiple templates of different sizes, designs, or styles.
Note: If you have layers with the same name, the API will not differentiate between them, and the same data will be sent to all similarly named layers within the Template Set.
Learn more about this by checking our article How to Auto Generate Multiple Image Sizes at Once ↗.