By default, all members of a role are considered "core members" of that role. That means those individuals are fully involved in the circle's governance decisions, can propose changes to their role and circle, may attend meetings, and so on.
However, there may be instances where it doesn't make sense for an individual member or an entire role to be involved in the decisions of the circle (typically if that member/role performs an insignificant task or doesn't spend much time involved in the circle). In these cases, the Circle Leader may opt for those individuals to not be considered core members. Being a "non-core member" just means you hold a role with work expectations but will not be directly involved in decisions made by the circle.