A subset template is an XML file that contains the details of the subset, consisting of the application, subset rules, rule parameters, and pre-scripts or post-scripts. When you create a subset definition and specify that you want to write subset data to export files, the export files become a template that you can subsequently import for reuse. You would import the template to perform subset operations on a different database.
Typically, the workflow is that you would first import a previously exported ADM template, which is another XML file, while creating an ADM. You would then import the related subset template while creating a data subset definition. You could alternatively select an existing ADM (skipping the import ADM flow) while importing the subset template.