Hi Meex,
This processing mode is the functionality provided for data buffering.
Here is the explanation for the different modes in the help document under data buffer section:
Synchronous Processing
The RFC is executed immediately, and the transaction waits for a response from SAP ECC. If an error occurs, the system does not execute the RFC again.
Asynchronous Processing on Error
The RFC is executed immediately. If the RFC cannot be executed because of network or connectivity issues, the action is added to the data buffer for future execution.
Asynchronous Processing
The RFC is not executed immediately, but it is added to the data buffer to be executed later. The transaction continues without waiting for a response from SAP ECC.
Here it is mentioned for RFC, but the same applies for http call also.
Regards,
Rohit Negi.