Emacs has at any time at most one selected tags table. All the commands for working with tags tables use the selected one first. To select a tags table, type M-x visit-tags-table, which reads the tags table file name as an argument, with TAGS defaulting to the first directory that contains a file named TAGS encountered when recursively searching upward from the default directory.
Emacs does not actually read in the tags table contents until you
try to use them; all visit-tags-table does is store the file
name in the variable tags-file-name, and not much more. The
variable’s initial value is nil; that value tells all the
commands for working with tags tables that they must ask for a tags
table file name to use.
In addition to the selected tags table, Emacs maintains the list of several tags tables that you use together. For example, if you are working on a program that uses a library, you may wish to have the tags tables of both the program and the library available, so that Emacs could easily find identifiers from both. If the selected tags table doesn’t have the identifier or doesn’t mention the source file a tags command needs, the command will try using all the other tags tables in the current list of tags tables.
Using visit-tags-table to load a new tags table when another
tags table is already loaded gives you a choice: you can add the new
tags table to the current list of tags tables, or discard the current
list and start a new list. If you start a new list, the new tags
table is used instead of others. If you add the new table to
the current list, it is used as well as the others.
You can specify a precise list of tags tables by setting the variable
tags-table-list to a list of directory names, like this:
(setq tags-table-list
'("~/.emacs.d" "/usr/local/lib/emacs/src"))
This tells the tags commands to look at the TAGS files in your ~/.emacs.d directory and in the /usr/local/lib/emacs/src directory. The order depends on which file you are in and which tags table mentions that file.
Do not set both tags-file-name and tags-table-list.