In the UV, a wealth of absorption features arises from the interstellar medium (ISM) observed toward hot stars, or from the circumgalactic/intergalactic medium (CGM, IGM, respectively) observed toward quasars, yielding fundamental insights on chemical abundances, kinematics, and polarization of the gas across a wide range of environments. Besides, the polarized UV band of AGN being an almost uncharted territory, Pollux will offer unique insight into the physics of AGN that is still little known, in particular by probing UV emitting and absorbing material arising from accretion disks, synchrotron emission in jet dominated.