The characters given by this option are included in identifiers. Identifiers are used in recognizing environment variables and after a match of the 'define' option. It is also used for "\i" in a |pattern|. See 'isfname' for a description of the format of this option. For '@' only characters up to 255 are used. Careful: If you change this option, it might break expanding environment variables. E.g., when '/' is included and Vim tries to expand "$HOME/.viminfo". Maybe you should change 'iskeyword' instead.