It can be both, an actual tag field, e.g., artist or a format string like %artist%. I've tried to clarify that in the note
- If
<field>in HAS/IS/GREATER/LESS/EQUAL/MATCHES contains$or%it should be enclosed in double quotation marks and will be treated as a format string instead of a tag field name. This will decrease search speed on large libraries since the format strings need to be evaluated before filtering.
A generating format string can be all of this:
And the same applies to that what is currently named <field> for filter.
Yes, but it's a little more tricky with the filter. What would be a text constant without spaces in a generating format string is a field reference in the Filter.
artist HAS test matches if any of the values in field ARTIST contains test, so it's not really treated as a text constant as in the context of format strings.
You can see the filter working with field names (not enclosed in %), which then uses value-lookup for the referenced field and uses the resulting string for matching. The extended functionality of the filter is allowing format strings instead of field names (anything with % or $ and enclosed in ") where the string is matched against the evaluated format string.
Admittedly, I did not consider this special function of the filter.
Yet, I think that the overlap between "generating format string" and the functions of the filter is larger than that of a field - as "field" is also used for the mappings and other references to fields and fields contents.
I agree that this behaviour has to be described but I disagree to call it a <field>.
I would not introduce a new meaning for "field" but use the note instead to decribe that additionally to the otherwise used reference to fields here, in this case, the % may be omitted (but even this is not mandatory).
I didn't realize this was possible even after all these years. My "learned something new" for today! 
Problems occur right now if older threads link to the old German help pages, like e.g. (my) post:
The original link intended to open the syntax reference for regular expressions which was formerly situated in the description of the action "Replace with regular expression".
https://docs.mp3tag.de/actions/replace-regexp/?de
This is now mapped to
https://docs.mp3tag.de/actions/#regexp
but it does not jump to the bookmark on that page and also, it does not show the regular expression which is now in a separate chapter:
https://docs.mp3tag.de/regex/
(which I think is better to have the regular expression syntax in 1 place as it can be used in the context of actions, scripting and filtering)
Yet, the old link is not correct any more.
Thanks, I'm aware of the issue. I've remapped all the help.mp3tag.de links in the forum to the new URLs, but had enough after one day of replacing strings directly in the database.
It's impossible to distinguish your special case, though. However, even if linked to Replace with Regular Expression – Mp3tag Documentation, it's only one step away from the syntax reference. The same applies to options_export.html, which was often used as reference for Format Strings and Placeholders – Mp3tag Documentation — but is now Exporting Tag and File Information – Mp3tag Documentation
I have to admit that I don't know how to untangle this Gordian knot.
Perhaps it is an idea to replace the links on a case by case basis.
Something different:
The Mac section has a dedicated "Documentation" category.
Will there be a "Documentation" category for MP3tag for Windows?
It's not planned at the moment, why are you asking?
I found the category quite appealing in the Mac section.
It shows openly the functions and features - and you don't have to "know" the address, it is publicly visible as part of the huge knowledge that has so far been accumulated around MP3tag.
You can see more details without the need to install MP3tag (to press F1 or call the help) if you can access the link to the documenation via the forum navigation.
I've added a table linked from the mapping section.
I went through it case by case 
I've now officially released the new documentation with Mp3tag v3.13. Thank you all, especially @ohrenkino, for your helpful and valuable feedback during the whole process of re-writing and re-structuring.
I think it's in very good shape now and much easier for me to keep updated. I've already noticed that it got easier to link to the documentation, because I know that it's not outdated or incomplete.
Here you go:
1] Why send the user to the website? What happens if the user's Internet connection is down?
2] Why remain old help files at C:\Program Files (x86)\Mp3tag\help\ when the first three entries in the Help tab send the user to the never version posted on-line?
3] How to access those locally stored help files from within Mp3tag? Which button or menu entry takes to that content?
A fair number of functions rely on the internet access - so now this also applies to the help. This is a change in function that the developer is entitled to do.
You may delete the help if you don't want it or you may keep it. The up-to-date and valid help is now online.
This function has been replaced with the access to the online help.
If you want to call the old help from within MP3tag, create yourself a tool for the html start page of the old, now outdated help.
Eee... my overall intention was to be able to use a dark themed version of the help files [Dark Theme missed elements - #7 by Zerow]. The only thing missing now is the offline [still dark] version of them. [And no: saving the HTML files and opening them from drive in the same web browser will not do the trick]
and
It would be great if the variables had their own labels as hyperlink target so that a link to the description of the variable would jump not only to the main page but directly to the section on the page where the description of a particular variable can be found.
All of those now have anchor IDs, e.g., https://docs.mp3tag.de/mapping/#popularimeter and I've now also added small link icons that appear when you hover over the heading.
I've also added links to all other headings, e.g., https://docs.mp3tag.de/format/#technical-information with the small link icon.
For the Tag field Mappings found here Tag Field Mappings – Mp3tag Documentation
can you include the old table at the end. I found it useful, and sometimes gave better clarity visually. If not, can you post a link to a file I can download and keep. Thanks for all your work on this!
On that page there is a link