Is Gii an extension or a module?

Yii 2.0 General Discussions
1

What I actually ask is, what the difference between extension and module is. When I saw Gii in " Official Extensions", I thought it must be an extension. But when I read the source code of yii\gii\Module, it says:“This is the main module class for the Gii module.” So, I got confused. If I want to build something myself. When should I build it as a module? And when should I build it as an extension? The Guide even don’t give definitions for them.

2

Everything that is build outside of core framework to improve the framework or add new functionality is an extension.

Module is a structure term. In Yii 2 it is described in the Guide.