Hi Omega/Aslam

Ideally (if possible to find out) we should transfer force, moment, movement & stiffness of that point in both the system.
It makes both the system as if combined system.

Here force & moment means interactive force of the system, when system is in join position. Both system produce effect on each other.
Generally it is not a easy task. That's why we can not break system arbitrarily.

We break system when header size is very big, branch size is quite small.
We assume branch is connected or not connected will produce same displacement in header calculation. In that case only displacement from header calculation, we can import in branch calculation. we can ignore force moment & stiffness.

If system is highly critical one, better to go for combined analysis.

Regards

Habib