So what's in it?
The SDK contains all the CICS TG JARs, RARs, shared libraries, headers, libs, and schemas that you might need to write, compile, and ship a CICS TG application for any edition of CICS TG V9.1. Whether you're writing a client application, implementing one of the exits, processing CICS TG statistics, or creating JSON web services, the SDK has what you need.
If you're developing a client application, the SDK includes the files needed to compile your application, samples showing how to use them, and for remote mode applications, the runtime files to deploy with the application. For example, a .NET developer can find relevant resources in cicstg/api/dotnet. This directory contains IBM.CTG.Client.dll to reference in Visual Studio when writing the application and deploy with the application, sample projects for EciB1 show how to write a basic COMMAREA application, and even the API documentation for reference.
Using one of the SPIs like CICS Request exits or the Java statistics API? The SDK provides the same kind of files as it does for client applications. So if you are developing a new Request Monitoring exit, look in cicstgsdk/exits/monitoring. You'll find ctgrme.jar, which contains the interfaces to implement and compile against, the BasicMonitor sample shows a simple implementation so you have an idea of where to start, and a local copy of the Javadoc is available too.
For creating JSON web services, look in cicstgsdk/webservices. This includes the JSON web services assistant and sample input files for creating WSBind files.
Finally the legal parts in cicstgsdk/license. The most interesting part is REDIST.txt as it explains what files can be redistributed, which is a very useful reference for understanding which files are needed when redistributing an application or just deploying an application on a remote machine without CICS TG installed.