DESIGNENERGY and Field map extension
According to the manual for OPAL <=1.6:
If the bend strength is set by K0 and/or K0S, then the actual beam will be bent a different angle if its energy does not correspond to the DESIGNENERGY of the bend
A beam energy different from DESIGNENERGY would change the bending radius and angle depending on K0. The beam trajectory matches always with the reference path inside the dipole.
The bending radius defines the extension of the field map. It is not clear to me if and how the redefinition of bending radius (due to a beam energy different from DESIGNENERGY) impacts on the field map extension.