An SDK is a combination of documentation and resources that are used for the development of tools and addons . Until now, the term “SDK” refered to the documentation for Dynamics CRM/365 and for the download package containing the assemblies and tools, and the offline version of the documentation. In order to clear up this confusion, and to be consistent with the terms for the Microsoft documentation, the developer documentation will not be refered to as the Developer Guide. This name is more descriptive for the content that supports developers in creating tools and addons using the available APIs.
Probably the greatest single change is that Microsoft will no longer be providing a download package for all the documentation, tools and sample code. Instead Microsoft will be offering an a-la-carte approach so that you can download the individual things as you need them. SDK assemblies and tools will be distributed only via NuGet. Microsoft will provide a script that will allow downloading the assemblies and tools from NuGet directly, and all the sample code will be available on msdn or on github.
The new documentation will be available on docs.microsoft.com, which will include documentation for the Client APIs, Web APIs, and more.
To read the full details regarding this release, see the Dynamics 365 blog on msdn: https://blogs.msdn.microsoft.com/crm/2017/11/01/whats-new-for-customer-engagement-developer-documentation-in-version-9-0/
Enjoy a new era.