Out-of-process Platform Extensions are extensions that run outside the Alfresco servers in its own processes.
The new event system in Content Services 7 enables out-of-process extensions. To support the event system a ReST API Java Wrapper is provided for easy development of extensions from Java applications.
You should get familiar with the SDK 5 as it is the recommended way of developing out-of-process extensions.
Read through the Getting started guide to get up to speed on the difference between in-process and out-of-process extensions.
To fully understand the platform extension points it is a good idea to read through the
Platform Architecture section.