In User Guide

What Is In?

In is an ExpressionEngine add-on. It does not have a user interface, and is intended as a development tool to be used in your templates. It allows you, as the developer, to Include template files (kind of like embeds, but earlier in the parse order), and to Insert files as early-parsed global variables (kind of like snippets, but using plain ol' templates).

Why In Is Awesome

It is an easy-to-use alternative to snippets. Unlike snippets, which are always loaded when EE runs, In only uses the templates when they are requested. Easy, performant, useful.