We’re talking about thermal, collisional/coronal, equilibrium, optically thin plasmas here…probably
Diagnostics and Physical Properties
Temperatures and overall emission levels: DEMs
Densities: line ratios…but also source location via f/i
Abundances: line ratios and line-to-continuum ratios
Kinematics: line broadening and profile shapes
Chandra (and XMM) have increased the spectral resolution available to X-ray astronomers by almost a factor of 100.