  There is a long history of using optical emission and absorption lines to
constrain the metallicity and ionization parameters of gas in galaxies.
However, comparable diagnostics are less well-developed for the UV. Here, we
assess the diagnostic potential of both absorption and emission features in the
UV and evaluate the diagnostics against observations of local and high redshift
galaxies. We use the CloudyFSPS nebular emission model of Byler et al. 2017,
extended to include emission predictions in the UV, to evaluate the metallicity
sensitivity of established UV stellar absorption indices, and to identify those
that include a significant contribution from nebular emission. We present model
UV emission line fluxes as a function of metallicity and ionization parameter,
assuming both instantaneous bursts and constant SFRs. We identify combinations
of strong emission lines that constrain metallicity and ionization parameter,
including [CIII] 1907, CIII] 1909, OIII] 1661,1666, SiIII]1883,1892, CIV
1548,1551, NII] 1750,1752, and MgII 2796, and develop UV versions of the
canonical "BPT" diagram. We quantify the relative contribution from stellar
wind emission and nebular line emission to diagnostic line ratios that include
the CIV 1548,1551 lines, and also develop an observationally motivated
relationship for N and C enrichment that improves the performance of
photoionization models. We summarize the best diagnostic choices and the
associated redshift range for low-, mid-, and high-resolution rest-UV
spectroscopy in preparation for the launch of the James Webb Space Telescope.
