The User Defined Report Maintenance option allows the user to customise reports to suit their needs. By Default each report has its own layout. For example the Client Listing report in Accounts Receivable Lists all of the Clients Names and Addresses. If you wanted to change the report in some way eg include the client’s phone number the User Defined Report Maintenance allows you to do this.
For more information see the Getting Started Manual – User Defined Report Maintenance.