Extension exposing the .NET CallContext class:

https://learn.microsoft.com/en-us/dotnet/api/system.runtime.remoting.messaging.callcontext?view=netframework-4.8.1

This can be used to store and access variables throughout the code execution path.

So, for example, in the OnBeforeRequest of an API, a 'CorrelationId' could be saved in the CallContext, and then later on in another Server Action further down the line, even in another module, that 'CorrelationId' could be fetched from the CallingContext.

This is useful in cases where you don't want to pass along certain variables through different layers of service actions, only to need the value at the end.