Let me help you out with the following piece of code assuming that we have a final code after being transclusion:
(If you want to know how this sample final code is achieved you can first check it out the link- Transclusion)
Some message from controller
This is from msg template
<div>This is inner message</div>
From above it is clear that there is only one controller named emp and directive named message in it which has a transclude text embed in it.
So, first the parent scope i.e controller scope is made available through the entire cycle and then we have directive scope which is named as directive isolated scope because the directive scope is independent from its parent scope. so, all the data available in controller scope is not available to directive scope and parent's data member is not available in directive scope untill we use $parent here which is used to access the data member of parent's scope i.e the controller scope
Now, we were having a transclusion scope which is having new instance independent of its parent scope and this particular instance is inherited from its parent scope . As it inherit the controller scope, so all data member is available in the transclusion scope and another special things to be remember when an new instance of transclusion is created there is always $parent member is associated with the transclusion scope along with parent's data member. In this case $parent is going to refer the directive scope but not to the controller scope. This $parent is used to access the data member of the directive scope as well as controller scope.
The whole scenario from the above explanation can be depict from the diagram attached below.