Why is both ./tags and tags present in the Vim 'tags' option? - vim

Question

If I execute :set tags? in Vim, I get the following output.
tags=./tags,./TAGS,tags,TAGS
If I execute the :help 'tags' in Vim, I get the following documentation.
*'tags'* *'tag'* *E433*
'tags' 'tag' string (default "./tags,tags", when compiled with
|+emacs_tags|: "./tags,./TAGS,tags,TAGS")
global or local to buffer |global-local|
Filenames for the tag command, separated by spaces or commas. To
include a space or comma in a file name, precede it with a backslash
(see |option-backslash| about including spaces and backslashes).
When a file name starts with "./", the '.' is replaced with the path
of the current file. But only when the 'd' flag is not included in
'cpoptions'. Environment variables are expanded |:set_env|. Also see
|tags-option|.
"*", "**" and other wildcards can be used to search for tags files in
a directory tree. See |file-searching|. {not available when compiled
without the |+path_extra| feature}
The |tagfiles()| function can be used to get a list of the file names
actually used.
If Vim was compiled with the |+emacs_tags| feature, Emacs-style tag
files are also supported. They are automatically recognized. The
default value becomes "./tags,./TAGS,tags,TAGS", unless case
differences are ignored (MS-Windows). |emacs-tags|
The use of |:set+=| and |:set-=| is preferred when adding or removing
file names from the list. This avoids problems when a future version
uses another default.
{Vi: default is "tags /usr/lib/tags"}
I want to know why both ./tags and 'tags` are present in this option?
The documentation says, When a file name starts with "./", the '.' is replaced with the path of the current file. So ./tags seem to behave similar to tags, or am I mistaken?
What is the difference between ./tags and tags in this option?

Answer 1

The path of the current file is not necessarily the same as the current working directory. With tags the latter directory is used, with ./tags, the former. The difference is explained in :h tags-option:
When a tag file name starts with "./", the '.' is replaced with the path of
the current file. This makes it possible to use a tags file in the directory
where the current file is (no matter what the current directory is). The idea
of using "./" is that you can define which tag file is searched first: In the
current directory ("tags,./tags") or in the directory of the current file
("./tags,tags").
For example:
:set tags=./tags,tags,/home/user/commontags
In this example the tag will first be searched for in the file "tags" in the
directory where the current file is. Next the "tags" file in the current
directory. If it is not found there, then the file "/home/user/commontags"
will be searched for the tag.
For example, consider the following directory tree:
.
├── project2
│   └── tags
└── project1
   └── tags
And you do:
cd project1; vim ../project2/foo.c
Then project2/tags will be used first, and then project1/tags (assuming you don't have autochdir set, or similar shenanigans aren't at work).