The Goal page is the first page of the Fitting Function Builder. You can choose to create or edit a User-defined fitting function in this page. Depending upon the chosen goal, subsequent pages of the Builder will display some minor differences.
The Fitting Function Library App lets you browse for add-on fitting functions. This App is pre-installed in the latest versions of Origin. If you don't find a particular fitting function within Origin's built-in library, check to see if it is available for download from the OriginLab website. Launch the App by (1) clicking on the Goal page link or (2) go to your Apps Gallery docked to the right side of your workspace and click on the App icon.