The diffuser is not adjustable via the setup menus, it is an integral part of the chassis design. I believe it can be adjusted in the BODYAERO section of the HDV file:
BodyFore=(0.0, -0.220, 0.443) // aero forces from moving forwards
Caster is in the "Suspension" section of the HDV file:
LeftCasterRange=(-2.0, 0.1, 81) // front-left caster
LeftCasterSetting=58
RightCasterRange=(-2.0, 0.1, 81) // front-right caster
RightCasterSetting=58
For suspension settings the first number is the base (in this case -2.0 deg.), next number is the incremental adjustment (steps of 0.1 deg.), third number is number of steps (81). Setting is the default within this range, here 58 of 81 (thus the 58th setting is -2.0).
But then, in HDV there is also the exclusive part of the Diffuser:
[DIFFUSER]
DiffuserBase=(-1.0246, 2.12, 83.74) // Base lift and 1st/2nd order with rear ride height
DiffuserFrontHeight=(1.450) // 1st order with front ride height
DiffuserRake=(-0.002, -20, 440.0) // Optimum rake (rear - front ride height), 1st order with current difference from opt, 2nd order
DiffuserLimits=(0.013, 0.105, 0.044) // Min ride height before stalling begins (0.0=disabled), max rear ride height for computations, max rake difference for computations
DiffuserStall=(0.1, 0.56) // Function to compute stall ride height (0.0=minimum, 1.0=average), downforce lost when bottoming out (0.0=none, 1.0=complete stall)
DiffuserDraftLiftMult=1.40 // Effect of draft on diffuser's lift response
DiffuserSideways=(0.432) // Dropoff with yaw (0.0 = none, 1.0 = max)
DiffuserPeakYaw=(1.5, 1.03) // Angle of peak, multiplier at peak
DiffuserCenter=(0.0, 0.01, -1.284) // Center of diffuser forces (offset from center of rear axle at ref plane)
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What I wanted is, like, how to show "Caster" or how to show "Diffuser" in the advanced part of the setup. I don't know if it's in this part of the HDV:
GarageDisplayFlags=255 // How settings are displayed in garage (add): 1=rear downforce value (vs. angle), 2=radiator (vs. grille tape), 4=more gear info, 8+16=front downforce/wing /splitter/air dam, 32+64=rear downforce/wing/spoiler, 128=flip ups (vs. fender flare)