Stacks 4 introduced some great new features, including 'Externals' and 'Templates'. Each of these allow stacks (or groups of stacks) to be preserved for use in multiple projects. With Source, we have included a number of templates within the Source stacks and these will automatically appear in the Templates section of your Stacks Library.
These are great to use as is, by simply dropping in your content. Or for 'unpacking' and applying your own stylings to. They are also really useful for learning about Source and how the templates have been put together and what settings have been used etc.