ESA is a technology similar to RFID as a technology.
For Ex: If a company does not have an SAP system at this moment. We have to work with their legacy system. Then we need to use the ESA. With the ESA, it will be easier to work with any system.ESA is used to support to enable RFID solutions.
Reference : TechEd '05 Vienna
SAP RFID Technology
¢ George Yu, SAP