Broadleaf is used to integrate with systems like SAP often. There are a number of approaches to integrating with SAP. The most common approach would involve mapping the SAP data and integrating it into the Broadleaf tables. As part of the mapping process, you will come across some incompatible structure changes and you can modify the Broadleaf structures to match the data from your source system.
Similarly, once a cart is submitted in Broadleaf, the common pattern is to queue the order and then run an integration process to push the order to the backend systems.
For promotions, we see two common approaches. If the promotions must originate in SAP, then, similar to product data, the promotion would be integrated into the Broadleaf offer tables so that cart pricing works as expected.
It is also common to use the Broadleaf promotion engine and then map the adjustments back into generic discount structures in SAP.