Core Roles still have work to be done, but the work to be done is more flexible and requires more coordination with other colleagues (of all role types). The ideal core role will still be fairly focused, though that focus tends to be more around an area or domain versus the more transactional, focused tasks of the SFR.
CRs are expected to have and take more ownership, too. They should be making decisions within the boundaries of their role (and sometimes outside of it, too). They can be the RP of projects, and coordinate others within those projects. They may have one or more Single Focus Roles helping with their work as well.
Core Roles (CRs) can be part or full-time, though the number of hours tends to be on the higher end at least when compared to the Single Focus Role.
Core Roles:
CRs vary a lot, and a lot of their definition should come on a role-by-role basis during role design.
A vast majority of roles at Tofugu are going to be either SFRs and CRs.