Let me start with What is scope?
A scope is an memory area or any logical area for storing data's or functions which is created when any controller, directive or Transclusion is instantiated.
So, There is no difference between Controller scope, Directive scope and Transclusion scope as each one having scope related to it but they differ only in inheritance and shared scope between them.
Now, let me take you to the senario i.e what is transclusion scope?
Let us consider the following piece of code:
Some message from controller
This is from template msg
<div>This is inner message</div>
From Above code it can be seen that there is a controller named emp, one directive named message and one transclusion inside the directive block.
So, when controller block in executed there is assign a special scope called controller scope and then control transfer to directive block and then another scope block is created. After creating directive scope, transclusion scope is created at the end where it can access all the data available in controller scope as well as directive scope.
For more clear to the topic here is the flow image:
Hope this helps you to understand about Transclusion Scope.