The dialog you use for advanced searches is mostly a matter of preference. The main functional difference between the two dialogs is that the Define Search dialog can be used to search for documents, folder, or work areas, whereas the Search by Form dialog can only be used to search for documents.

If you have a preference for one dialog over the other, you can set it as the default advanced search dialog. This dialog will open whenever you select Folder > Advanced Search, or when you click the Advanced Search icon in the Search toolbar. If you do not set a default advanced search dialog, then you will always be prompted to select one of the two dialogs.