There is an Requirement :-
In one sales order can we do ATP check in ECC & APO for
2 different material, one is APO relevant Material ( RBA active)
& other is ECC relevant material .
APO relevant material RBA is actived & For Sales order type business transaction RBA is maintained.
Create Integration Model (CFM1) Transaction - Create IM for ATP Check after reading below SAP Help details -
Availability Check in Integration Model Use This indicator can be used to activate Available-to-Promise (ATP) check for the selected material-plant combination for the SAP APO system entered under Target System. This means that ATP checks that you trigger for this material-plant combination in the ERP system will, in future, be executed in SAP APO. If the ATP Check indicator is not set for a certain material-plant combination, ATP checks for this combination that you trigger in the ERP system will also be carried out in the ERP system. ATP checks that you trigger in SAP APO are not affected by this setting. Dependencies The ATP check must be activated before or together with the transfer of sales orders in an integration model, otherwise ATP-relevant data is missing in SAP APO.
Also please check CO09 is accessing your APO system for the material/plant for which ATP Check is active and for others it should open screen from your ERP System only
Please let me know if you still have any queries.