  The J-region Asymptotic Giant Branch (JAGB) method is a standard candle based
on the intrinsic luminosities of carbon stars in the near infrared. We directly
constrain the impact of metallicity, age, and reddening on the JAGB method. We
assess how the mode, skew, and spread of the JAGB star luminosity function
change throughout diverse stellar environments in M31's NE disk from 13<d<18
kpc using data from the Panchromatic Hubble Andromeda Treasury (PHAT). As
expected, the mode is found to be fainter in higher-reddening regions. To
cross-check this result, we also measure a fiducial J-band ground-based JAGB
distance using data from the UKIRT/WFCam in M31's outermost disk (18<d<40 kpc)
where internal reddening is minimal. We find that this J-band distance modulus
agrees well with the F110W distance moduli measured in the lowest reddening
regions of the PHAT data, demonstrating the JAGB method is most accurate if
measured in the low-reddening outer disks of galaxies. On the other hand, the
mode of the JAGB star luminosity function appears empirically to show no
dependence on age or metallicity within the range -0.18<[M/H]<-0.26 dex. In
conclusion, the JAGB method proves to be a robust standard candle capable of
calibrating the luminosities of type Ia supernovae and therefore providing a
high-accuracy, high-precision measurement of the Hubble constant.
