  The formation of the transition-region O IV and Si IV lines observable by the
Interface Region Imaging Spectrograph (IRIS) is investigated for both
Maxwellian and non-Maxellian conditions characterized by a kappa-distribution
exhibiting a high-energy tail. The \ion{Si}{4} lines are formed at lower
temperatures than the O IV lines for all kappa. In non-Maxwellian situations
with lower kappa, the contribution functions are shifted to lower temperatures.
Combined with the slope of the differential emission measure, it is possible
for the Si IV lines to be formed at very different regions of solar transition
region than the O IV lines; possibly close to solar chromosphere. Such
situations might be discernible by IRIS. It is found that photoexcitation can
be important for the Si IV lines, but is negligible for the O IV lines. The
usefulness of the O IV ratios for density diagnostics independently of kappa is
investigated and it is found that the O IV 1404.78A /1399.77A ratio provides a
good density diagnostics except for very low T combined with extreme
non-Maxwellian situations.
