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 in the default directory as the default.
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 setting the variable
yourself is just as good. The variable's initial value is
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:
(setq tags-table-list '("~/emacs" "/usr/local/lib/emacs/src"))
This tells the tags commands to look at the TAGS files in your ~/emacs 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