Not sure how much we can help here as I think most of this would be an ATP questions. It looks like you made a similar posting in the ATP area. Please make sure you let us know if you get a solution from there and than close this thread out. I can't help you with the gATP and the ECC ATP differences.
As far as the QM part of your question: The 89 lot will get created automatically at GR time in the receiving plant provided 89 is activated for the material in the receiving plant.
The 89 set up in the sending plant has no affect.
.What APO SHOULD be looking at to figure out availability date should be the expected GR date fot the STO PLUS the GR processing time.
This means that the GR processing time should be at least the same as, or greater than the average inspection duration used in the 89 lot. (this can cause conflicit with 01 and 04 lots if you have more than one way of getting the material).
Once the 89 inspection lot is created, APO will then use the end date of the created inspection lot. As a result, unless you have carefully considered the GR processing times and average inspection duration when you set up the material, there is a good chance your availability date in ATP will be changed after the GR of the STO.
So.. for instance.. GR processing time = 9 days.
89 average inspection duration = 7 days.
An STO is created, expected delivery date is calculated to be Jan 15, 2016.
APO should see an availablity of Jan 24, 2016 (Jan 15 + 9 days) and no inspection lot info yet.
On Jan 15, the shipment arrvies, after GR the inspection lot is created and inspection lot end date is current date + avg. inspec. duration = Jan 24, 2016.
If however, you failed to properly account for inspection time in the GR processing time you get the below scenario.
GR processing time = 1 day
89 average inspection duration = 7 days
Expected delivery date is Jan 15, 2016.
APO sees an availability of Jan 16. (Jan 15 + 1 day of GR processing time)
But when the GR is down, the insepction lot is created with an end date of Jan 22. (current GR date of Jan 15 + AID of 7 days).
Now APO of the material changes from an original planned date of Jan 16, to Jan 22.
I hope this helps.
Craig