Although the IUE data set does not represent a uniform survey of the sky, the large variety of objects observed by it offers unique opportunities to derive ``average'' properties of celestial populations. This has been used by many (e.g. Fanelli et al. 1987) to derive UV-to-optical color indices for various stellar spectral types and luminosity classes. These are later used to derive transformations, to create models of the UV sky (Brosch 1991), or to determine the level of the diffuse UV background. Observations of galaxies were used to determine average UV spectra of irregular, spiral, and elliptical galaxies, and of galactic bulges (Ellis et al. 1982, Burstein et al. 1988, Kinney et al. 1993, Storchi-Bergmann et al. 1994), important for the derivation of cosmological k-corrections, and for analyzing observed properties of high-redshift clusters of galaxies (Steindling et al. 2000).
To help the logical usage of the information collected by IUE, atlases of UV spectra of selected types of objects, based on IUE data were published (i.e. Longo & Capaccioli 1992 for normal galaxies, or Courvoisier & Paltani 1992 for active galaxies). Apart from these special-purpose atlases, note those dedicated to the classification of stars from their UV spectra published by ESA and by NASA (Heck et al. 1984; Wu et al. 1991).