The report looks for unused App Service Plans by checking if each App Service Plan holds a relationship to an App Service.
If the plan is not related to any App Service at the time of looking it is considered unused. The report looks at these assets like a snapshot in that point in time. It is useful to use the “last modified” column to determine how long and when the last time that ASP was updated and potentially associated with an App Service. As an example, an ASP with a modified date in the last day or two might not be as reliable as an ASP that has not been modified in a month and is sitting with no related App Services.