S4H Side-by-side 开发
系统外开发，然后通过白名间的API集成回来到S/4 HANA Cloud,比如在客户在开发系统进行开发，或者在SAP Cloud Platform开发，然后集成回来到S4 Hana Cloud
A2A B2B
引用：https://answers.sap.com/questions/4229678/what-is-a2a-and-b2b.html
A2A means you are communicating within same firewall or in same company or same land scape.
For example, if One company ABC has implemented the R/3 & XI and say there will be serveral departments (modules like Sales, Material Mgmt etc…) So if Sale department is dealing with the Material management department within the company then you will say it A2A.
B2B mean communicating outside the firewall.
For example there is some data coming form third party which is not in the same landscape then you have to use third party adapter like Sea burger, AS2, X400, OFPTS, FILEVAN.
b2b scenarios involve exchange og msgs across diff organizations. this involves concept of parties (representing the organizations).
a2a are application to application scenarios. they involve exchange of messages between diff systems/depts within “one” organization. the systems in your landscape need not belong to any party!
A2A means application to application,it refers to scenarios where both the sender and receiver and with ur landscape,suppose ur doin a File to IDOC scenario where in both the FTP server and the R/3 server is within ur landscape then it will be called A2A,in most A2A cases we use business system and service without party.
B2B means busines to business,it refers to scenario where both the sender and the receiver and in different landscape,suppose ur again doing a File to IDOC scenario but the FTP server is in a separate landscape(for e.g ur clients site or anywhere else) and the R/3 server is in ur landscape.we have to use Party for this to identify separate business.we also prefer Business service over business system in this case.
Differences
A2A communication is required whenever you are sending messages within the same landacapes ie within the corporate boundary .In this case you have to use Service without party in ID.For B2B you have to use Party in ID.