combining multiple service into one service

hi

i am trying to use one standard service and two external services to build a custom service. how do i use modelling toll available in PI 7.1 to do it. by the i really do not understand the concept of creating models. i mean the need for creating a model in PI 7.1.

i want to build the service in ABAP not in NWDS7.1 using composite applicaiton framework

thanks