This dialogue is opened whenever you indicate that either:

 

 

Select a project template from the list of known templates and press OK to confirm the selection.  If you select a template for a project that either receives or emits data of a type or types that have not been predetermined by the template, the Type Browser will be shown to allow you to specify one or both data types.  If you are unsure which template to use, select Project (as shown above).  You can always change the project type later by selecting Project → Change Project Type... from the menu bar.