Hi @Learner-in-Prog , I am afraid your question is quite broad in nature.
If you are looking for integrating SAP with OMT as part of the fulfilment, you can explore Integration Spokes available OOTB to see if they fit your use case. Check for SAP on ServiceNow Store.
If none of the Spokes meet your needs, you can build your own REST Action(s). There is a developer guide that takes you step by step through this.
Once you have identified the Spoke and or built the REST action, this will be available in the Flow Designer Actions and can be used in the Subflows that you will invoke against the respective Domain Orders/Specs.
Can you elaborate more on the Staggered Decomp question?
Staggered decomposition will happen automatically based on decomposition rules defined in the Catalog and subject to the conditions getting satisfied. If your question is on how to enrich the order for the decomposition that is paused (as part of the staggered process) - then you can either auto update the characteristics via Subflows if there is a dependency on 3rd party data or manually enrich it via Order Tasks.
Regards
Shashank
