What limits the field, live: the magnetic pressure at the coil bore drives a hoop stress and a peak fiber stress in the REBCO tape that climbs as B². Push the field and the peak fiber stress approaches the REBCO allowable (~660 MPa) — that is what caps the magnet. Surrogate; ANSYS / COMSOL retire it.
Honest gate The peak fiber stress must stay below the REBCO allowable (~660 MPa, the irreversible-strain limit). At the design field it has margin; push B and it rises as B². This reduced-order elastic model is what ANSYS / COMSOL full 3-D FEA retire; the real magnet also has winding, insulation and quench structure.