A shell star (indicated by the spectral type suffix, sh) is a type of star showing both broad and narrow absorption lines. The assumption has been they are rotating rapidly, causing the broad lines, and they are surrounded by glowing material (a surrounding "shell") producing the narrow lines. They are an object of research interest and this assumption has been and is being refined. The "shell" is now presumed to be a circumstellar disk, in particular a decretion disk, with material decreted by the rapid rotation. The disk becomes approximately Keplerian, much less radial-velocity variation than the star and its spectral lines are narrow. Some of the stars have Balmer series emission lines and are classified as Be stars, and there is now a general notion that these are phases which at least some B-type stars pass through. The shell star and emission-line characteristics are most common in B-type stars but are found among O-type through F-type stars. Rapid rotation is suspected to be the cause of such evolution, or at least a factor. A factor in their appearance to us is the angle of view we have of the star: the notion eventually developed that viewing the disk edge-on, or at an angle, or viewing the star pole-on could explain a number of different types of odd stars. The stars also have variable characteristics (including appearance/disappearance of the emission lines and the narrow lines, i.e., the defining features of shell stars and Be stars). One variable-star classification considered to be the same general sort of star is the Gamma Cassiopeiae variable (GCAS).
I have seen one source speaking of Balmer series lines from high electron shells, worded almost to suggest the term "shell star" was due to these electron-shell characteristics.