Subforms are a way to use the content of a form file within or as part of the structure of a main form in a form group. They can be used multiple times in one or more forms to repeat or "share" information.  This makes them useful for saving time when making or updating forms, since updating the contents of the form used as a subform automatically can update all areas where it is used or shared. Subforms are often used to replicate identical rooms such as bathrooms or bedrooms, or other components where a report may need indentical repeated sections.  Since many items can be replicated on the fly as you inspect, it's good to only include the minimum number of items always used directly in the master forms to keep them as small and efficient as possible.