The "complete" template lets you install everything. The default folder structure, all the scss files, and code to get you started (mixins, functions, variables, etc).
Learn more about the available helper code.
The "clean" template lets you install the same default folder structure minus the code inside. This is particularly useful, if you want to start fresh, and do not need any snippets.
The "custom" template lets you choose the structure you want. Choose between the available folder and create the scss structure you need.