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Example - You want to track devices that are licensed under Cisco Smart Net Total Care; to do this you want to include 2 fields - a listbox with Yes/No and a date field to store the date of expiration. These fields should only be assigned to Device in the Routers Group.
Reporting
You can include custom Fields in Queries, but not Reports or Summaries. The general process for for creating a custom report is covered HERE: Creating Custom Reports, with details on using custom Fields covered in depth HERE: Create a Query containing Custom Fields