Monica,
One way to "categorize" roles/activities is by knowing your goal i.e. become a SAP consultant.
A core team member is someone who knows the business processes and is a liaison between the business users and the consultants. He/she does not know the particular ERP system.
A superuser knows the ERP system, in this case SAP and is the first point of contact for the business users, when they face issues in SAP.
To become a consultant, it is a long way from superuser (and longer way from core team member). You have to be an expert in a module in SAP, have configuration knowledge, experience and business process knowledge. So that is the part which the consultant is advising with "more study".
TW