M-x list-tags reads the name of one of the files described by the selected tags table, and displays a list of all the tags defined in that file. The "file name" argument is really just a string to compare against the file names recorded in the tags table; it is read as a string rather than as a file name. Therefore, completion and defaulting are not available, and you must enter the file name the same way it appears in the tags table. Do not include a directory as part of the file name unless the file name recorded in the tags table includes a directory.
M-x tags-apropos is like
apropos for tags
(see section Apropos). It reads a regexp, then finds all the tags in the
selected tags table whose entries match that regexp, and displays the
tag names found.
You can also perform completion in the buffer on the name space of tag names in the current tags tables. See section Completion for Symbol Names.