SAP Process Orchestration 7.4 can installed in three different options:
If PI is installed as AEX, PO can be installed using the same system ID (SID), meaning PI, BPM and BRM are installed on one JEE server. This will improve TCO due to savings in hardware, support and administration. There are also performance improvements due to internal processing of messages between PI and BPM/BRM.
AEX can also be installed by itself on one SID, with BPM/BRM installed on a different SID. With this option, BPM/BRM cannot be added to AEX at a later time; a new installation will be required. (In my opinion, this is not recommended for a PO license unless there is a special reason to do so. However, this might be a valid path for PI volume-based license as stated below.)
If PI is installed as dual-stack, then PO, PI and BPM/BRM, will have to be installed with different SIDs. PI on one SID and BPM/BRM on a different SID.