What does “tag” mean in the context of Vim's help window?

Question

I ran :help index to find out about key mappings and it's great, but there's one thing I don't get. At the top it says...

This file contains a list of all commands for each mode, with a tag and a short description.

Indeed, the leftmost column of information in each section is "tag".

So e.g., the first thing is...

tag             char            action in Insert mode
-------------------------------------------------------------------------
i_CTRL-@        CTRL-@          insert previously inserted text and stop

What is the nature and purpose of the tag i_CTRL-@?

Does it do something useful I should know about?

Answer 1

The tag is useful for two things:

1. When using the :help command, the tag is the exact phrase which will let you distinguish between similar terms.

                                                   {subject} E149 E661
   :h[elp] {subject}       Like ":help", additionally jump to the tag {subject}.
   

   For example, <c-A> is available in insert, normal and command-line modes. To get to the help entry for:

   • insert mode, do :h i_CTRL-A.
   • normal mode, do :h CTRL-A.
   • command-line mode, do :h c_CTRL-A.

2. You can use them for tag navigation. From :h tags:

   With the ":tag" command the cursor will be positioned on the tag.  With the
   CTRL-] command, the keyword on which the cursor is standing is used as the
   tag.  If the cursor is not on a keyword, the first keyword to the right of the
   cursor is used.
   

   So, when on a tag, you can press <c-]> to jump to where the tag is defined, which is where the full help for that entry will be present. For example, in the list you show, if you move to i_CTRL-A and press <c-]>, you'll get to:

                                                   i_CTRL-@
   CTRL-@          Insert previously inserted text and stop insert.  {Vi: only
                   when typed as first char, only up to 128 chars}
   

   And you can press <c-T> to jump back to where you were.

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See also: points 6 and 9 in this excellent answer about the help system.