The characters specified by this option are included in file names and path names. Filenames are used for commands like "gf", "[i" and in the tags file. It is also used for "\f" in a |pattern|. Multi-byte characters 256 and above are always included, only the characters up to 255 are specified with this option. For UTF-8 the characters 0xa0 to 0xff are included as well. Think twice before adding white space to this option. Although a space may appear inside a file name, the effect will be that Vim doesn't know where a file name starts or ends when doing completion. It most likely works better without a space in 'isfname'.