Structure for the design functional tuple = {"Functional Name": "Expression"}. "Functional Name" must be one of the SU2 pre-defined functionals:
DRAG, LIFT, SIDEFORCE,
MOMENT_X, MOMENT_Y, MOMENT_Z,
EFFICIENCY, BUFFET,
EQUIVALENT_AREA, NEARFIELD_PRESSURE,
FORCE_X, FORCE_Y, FORCE_Z, THRUST,
TORQUE, TOTAL_HEATFLUX, CUSTOM_OBJFUNC,
MAXIMUM_HEATFLUX, INVERSE_DESIGN_PRESSURE,
INVERSE_DESIGN_HEATFLUX, SURFACE_TOTAL_PRESSURE,
SURFACE_MASSFLOW, SURFACE_STATIC_PRESSURE, SURFACE_MACH

and "Expression" must be an empty string.

If "Functional Name" is "CUSTOM_OBJFUNC" then "Expression" defines the objective which is parsed symbolically, e.g.:

{"CUSTOM_OBJFUNC": 'DRAG + 10 * pow(fmax(0.4-LIFT, 0), 2)'}