Emacs has at any time at most one selected tags table. All the commands for working with tags tables use the selected one. 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.
visit-tags-table when a tags table is already loaded gives you a choice: you can add the new tags table to the current list of tags tables, or start a new list. The tags commands use all the tags tables in the current 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 strings, like this:
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