1
The third uncertainty is due to the uncertainty in the ${{\mathit \Xi}_{{{c}}}^{+}}$ mass, taken to be the PDG 2022 fit result $2467.71$ $\pm0.23$ MeV.
2
The third error is the uncertainty on the ${{\mathit \Xi}_{{{c}}}^{+}}$ mass. (AAIJ 2017AH gave ${}^{+0.3}_{-0.5}$ MeV here, but as of 2018 it is $\pm0.3$.)