This option only works if you already have an open file.  When selecting new input file , you choose File > New Input File.  A pop-up window appears with three options, Yes, no and Cancel.

• By selecting Yes, you delete all data from that file and a new window appears where you can Select New Unit of Analyse .
• By selecting No, you can also Select New Unit of Analyse but you don´t delete the data in the file.  When you decide to save your work later, you overwrite all former data in the file.  If you on the other hand want to keep the file unchanged you simply do not save your work.