Side-By-Side HCM enables organizations to deploy SuccessFactors Employee Central (EC) as the Core HR system for a specific population of employees, whilst retaining SAP ERP HCM for the other set of employees.
Recently, there has been an increased adoption of this deployment model by organizations as this offers flexibility in their adoption of cloud technologies.
With the recent enhancements from SAP, now it's really easy to set up this deployment model. The video here describes how to set up the Side by Side Model. It includes EC configurations, Boomi settings, SAP ERP configurations and the actual data replication runs.