说明：后面所有相关文章原文皆为3DSlicer官网所载，进行了摘录并添加自己的理解，便于后面的开发者快速理解3DSlicer开发的套路，另外觉得官网的原版英文还是原汁原味的好，直接翻译过来可能就变味了，因此以中英文对照形式学习可能更好。
What is an extension ?
An extension could be seen as a delivery package bundling together one or more Slicer modules. After installing an extension, the associated modules will be presented to the user as built-in ones.
The Slicer community maintains a website referred to as the Slicer Extensions Catalog to support finding, downloading and installing of extensions. Access to this website is integral to Slicer and facilitated by the Extensions Manager functionality built into the distributed program. (Slicer社区维护着一个称为Slicer Extensions Catalog 的网站，用户可以在该网站上发现、下载、安装相应的extensions来满足用户的功能需求，当然，Slicer这个软件直接在软件中集成了这个网站的功能，通过3DSlicer中的Extensions Manager ，用户可以直接在软件中获取到该网站上的各种extensions，并可以直接进行下载安装，十分方便)
The Catalog classifies extensions into three levels of compliance: （ Catalog将所有的extensions分为三个兼容性级别）
- Category 1: Fully compliant Slicer Extensions: Slicer license, open source, maintained.
- Category 2: Open source, contact exists.
- Category 3: All other extensions (work in progress, beta, closed source etc).（不想公开源码的）
To publish extensions, developers should consider reading the following pages:
Extensions in the Extension manager.
来自 <https://www.slicer.org/wiki/Documentation/Nightly/Developers/FAQ#What_is_an_extension_.3F>