[en] In view of that the diffuse component of soft X-rays is emitted mainly by extended supernova remnants and the hot interstellar medium produced thereby, the following three points are discussed. (1.) In the early stage of supernova remnants expanding in low density media, neither the equipartition of energy between electrons and ions nor the ionization equilibrium holds. The shock structure is modified by the pressure of hot plasma. X-ray features of the North Polar Spur are interpreted by taking these points into account. (2.) Several compilations of X-ray line emission are compared. Those by Kato and by Raymond and Smith are different in the equilibrium ion abundances and the collisional excitation rates for some important lines. The origins of differences are shown for some examples. It is demonstrated how the difference affects astrophysical interpretations. (3.) The solar system is surrounded by a hot, tenuous interstellar medium extended in the direction of 1 -- 150⁰ in the northern hemisphere. In other directions X-rays emitted farther than absorbing clouds are appreciable. (author)