It means the main thing is that.
You've created a standard PO for plant A, then you are using a STO to transfer the material from plant A to plant B.
Now you are trying to find a relation between standard PO and GR for STO PO.
I don't think there are any standard relation between these process.
As you are using two different complete process. First you've created a PO and done GR. Its a complete different process and then you have done STO process which is a different complete process.
You might have to go for a development to find your won relation between these two process.