One of the thing you want to use as an included template files is mostly 2nd level menu (sidebar menu).
Include files must be in themes/yourtemplatename/templates/Includes folder it’s the same level as Layout folder.
2nd level menu example code u add to themes/yourtemplatename/templates/Includes/Menu2.ss:
<% if Menu(2) %> <ul id="Menu2"> <% control Menu(2) %> <li class="$LinkingMode"><a href="$Link" title="Go to the $Title page">$MenuTitle</a></li> <% end_control %> </ul> <% end_if %>
And to include the Menu2.ss use in your themes/yourtemplatename/templates/Layout/Page.ss code:
<% include Menu2 %>