Here is a solution for the first part of my question. This is based on
https://www.mail-archive.com/ntg-context@ntg.nl/msg101156.html
and try and error based on publ-imp-aps.mkvi and publ-imp-apa.mkvi:

It display now the citation _and_ numbering labels in the short form:
```
\setupinteraction[state=start]

\startbuffer[testdata]
@Book{knuth1,
	author = {Donald E. Knuth},
	title = {TEX and METAFONT. New directions in typesetting},
	year = {1979},
	publisher = {Addison-Wesley},
}
@Article{someother,
	author = {Some Person and Some Otherperson},
	title = {Another title},
	year = {1900},
}
\stopbuffer

% enable tracing
\enabletrackers[publications, publications.crossref, publications.details, publications.cite, publications.strings]

\usebtxdataset[main][testdata.buffer]
\usebtxdefinitions[aps]
\setupbtx[dataset=main]
\definebtxrendering[bibrendering][aps][dataset=main, numbering=short]
\setupbtxlist[aps][
  alternative=b,
  distance=.5em,
]
\setupbtx[aps:cite][alternative=short]

\starttext

Knuth developed \TeX \cite[knuth1]
Several persons said something \cite[someother].

\placelistofpublications[bibrendering]

\stoptext
```

The remaining part now is to mark publications of Knuth in a different way.
I have tried this and failed. Here is my code so far:
```
\startluacode
function render_cite(short, author)
	print(author)
	if author:lower():find("knuth") then
		context("[>" .. short .. "]")
	else
		context("[" .. short .. "]")
	end
end
\stopluacode
\definebtx[aps:cite:special][aps:cite]
\startsetups btx:aps:cite:special
	\btxcitereference
	\ctxlua{render_cite([==[\btxflush{year}]==], [==[\btxflush{author}]==])}
\stopsetups
% use it with \cite[special][knuth1]
```
This has several problems:
- \btxflush{short} does not work as I expected that. How can I get the short form?
- \btxflush to pass arguments to lua does not work as intended. On the lua side the
   variable "author" resolves to "\btxflushauthor{author}" and not to "D. E. Knuth".
   How can I pass the data?

I use Lua only because I find it more convenient as a programming language. If a
plain tex solution is simpler, I'm happy to use that. 


Another question that came up while studying the source code:
What effect have \c!, \s!, and \v!? These commands exist a lot in the above
cited files.

Gerion


Am Mittwoch, 17. April 2024, 15:44:12 MESZ schrieb Gerion Entrup:
> Hi,
> 
> > The APS style is an example of a number-based citation system. As you
> > are looking for a tag-based citation, it might be better to start with
> > something like the APA style.
> 
> I'm kind of satisfied with the publication-list layout of the APS style so I chose
> this as a base. If it is simpler to start with the APA style, I can do that.
> 
> My hope is that it is possible to just change the "label" that is used for
> referencing and keep everything else as is.
> 
> > Is there some specification for your short tag-based citation style?
> 
> biblatex implements this with the "alphabetic" style. I'm not aware of
> a specification, though. Does the "short" form in ConTeXt exist for a
> specific reason? :)
> 
> Gerion
> 
> > On Tue, 16 Apr 2024 06:04:35 +0200
> > Gerion Entrup <gerion.entrup@flump.de> wrote:
> > 
> > > Hi,
> > > 
> > > I'm trying to customize a cite format in a bibliography. Currently,
> > > I'm using the aps style which fits so far, except of the plain number
> > > as reference. In concrete, I want to change the following:
> > > - Use the first letter of the last name and the year as reference
> > > (this should be exactly the short form). It is more less also
> > > described here [1].
> > > - Highlight publications of a certain author with an extra char (e.g.
> > > '>')
> > > 
> > > Here is a minimal example (lets assume, every occurrence of Knuth
> > > should be highlighted): ```
> > > \setupinteraction[state=start]
> > > 
> > > \startbuffer[testdata]
> > > @Book{knuth1,
> > > 	author = {Donald E. Knuth},
> > > 	title = {TEX and METAFONT. New directions in typesetting},
> > > 	year = {1979},
> > > 	publisher = {Addison-Wesley},
> > > }
> > > @Article{someother,
> > > 	author = {Some Person and Some Otherperson},
> > > 	title = {Another title},
> > > 	year = {1900},
> > > }
> > > \stopbuffer
> > > 
> > > % enable tracing
> > > \enabletrackers[publications, publications.crossref,
> > > publications.details, publications.cite, publications.strings]
> > > 
> > > \usebtxdataset[main][testdata.buffer]
> > > \usebtxdefinitions[aps]
> > > \setupbtx[dataset=main]
> > > \definebtxrendering[bibrendering][aps][dataset=main]
> > > 
> > > % this seems to have no effect?
> > > \setupbtx[alternative=short]
> > > 
> > > \starttext
> > > 
> > > Knuth developed \TeX \cite[knuth1].
> > > Several persons said something \cite[someother].
> > > 
> > > \placelistofpublications[bibrendering]
> > > 
> > > \stoptext
> > > ```
> > > 
> > > It should be rendered as:
> > > ```
> > > Knuth developed TeX [>Knu79].
> > > Several persons said something [PO00].
> > > 
> > > [Knu79]	D.E. Knuth, TEX and METAFONT. New directions in
> > > typesetting (Addi­son-Wesley, 1979). [PO00]	S. Person and S.
> > > Otherperson, Another title, (1900). ```
> > > 
> > > How can I achieve that \cite[something] always renders in the short
> > > form by default? How can I customize that references to Knuth contain
> > > this extra '>'? How can I customize the APS style to also use the
> > > short form in the list of publications instead of numbers?
> > > 
> > > Best
> > > Gerion
> > > 
> > > [1]
> > > https://tex.stackexchange.com/questions/295444/citation-style-with-first-letters-of-authors-lastname