In Plex Web you can hold Ctrl to "multi-select" for Movies, TV shows, and Artists. However, this seems to only apply to the highest level of the libraries because you cannot do the same for TV episodes/seasons or music albums/artists. I would love to see this feature expanded so that I can multiselect at arbitrary levels of the library. This would be particularly useful when marking only part of a tv show as "Watched" or adding specific songs to a playlist.
This may apply to other clients as well but I mostly only use web so thats the only one I am sure of.
