Core SAP Business Modules

ECC and R/3 are composed of business modules. 

Each and every module does not have to be deployed.

For example, if your objective is to efficiently manage financial accounting, controlling, and treasury cash management, there there is no need to implement human resources module and manufacturing module. You only need to deploy SAP Financials module.  There could be other systems in place to take care of human resources and manufacturing processes.

SAP can be easily constructed to interface with the other systems.