The scope property takes a single string setting that is the “path” of the sub-part of the pipeline that should go to a service.
For instance if the top level of your pipeline has lots of stuff in it, but all the parameters for myFolder:myService are in the Record myParams, which is in the root of the pipeline, set the scope value to /myParams.
myService would see only the contents of myParams (not the containing myParams record, so it must be addressing the children of myParams directly).
This can increase performance because the Flow Engine does some shallow cloning of the pipeline in order to implement some basic pipeline rollback behavior. A shallow clone of a big pipeline that is not used by the called service is a waste.