  We use 3D radiation magnetohydrodynamic models to investigate how the
thermodynamic quantities in the simulation are encoded in observable
quantities, thus exploring the diagnostic potential of the 133.5 nm lines. We
find that the line core intensity is correlated with the temperature at the
formation height but the correlation is rather weak, especially when the lines
are strong. The line core Doppler shift is a good measure of the line-of-sight
velocity at the formation height. The line width is both dependent on the width
of the absorption profile (thermal and non-thermal width) and an opacity
broadening factor of 1.2-4 due to the optically thick line formation with a
larger broadening for double peak profiles. The 133.5 nm lines can be formed
both higher and lower than the core of the Mg II k line depending on the amount
of plasma in the 14-50 kK temperature range. More plasma in this temperature
range gives a higher 133.5 nm formation height relative to the Mg II k line
core. The synthetic line profiles have been compared with IRIS observations.
The derived parameters from the simulated line profiles cover the parameter
range seen in observations but on average the synthetic profiles are too
narrow. We interpret this discrepancy as a combination of a lack of plasma at
chromospheric temperatures in the simulation box and too small non-thermal
velocities. The large differences in the distribution of properties between the
synthetic profiles and the observed ones show that the 133.5 nm lines are
powerful diagnostics of the upper chromosphere and lower transition region.
