Actually the issue was discussed some time ago and Wolfgang Schuster explained that.
In fact after sending you the previous answer I realized that he explained also that the right way is to use \defineconversionset and then reset the userpagenumber when necessary: look at the following example:

%%%% begin example
\defineconversionset[frontpart:pagenumber][][romannumerals]
\defineconversionset[bodypart:pagenumber] [][numbers]
\defineconversionset[backpart:pagenumber] [][characters]

\starttext
\startfrontmatter
\input knuth.tex
\stopfrontmatter

\startbodymatter
\setupuserpagenumber[number=1]
\input ward.tex
\stopbodymatter

\startbackmatter
\setupuserpagenumber[number=1]
\input tufte.tex
\stopbackmatter
\stoptext
%%%% end example

It would be nice to wikify things each time one learns something…

Best regards: OK

On 22 May 2014, at 12:24, Werner Hintze <we.hintze@gmail.com> wrote:

> Thank you! This works.
> 
> But tell me: How do you know this? The manual and the website don’t explain it this way…
> 
> Best regards
> 
> Werner
> 
> On 22 May 2014, at 12:16, Otared Kavian wrote:
> 
> Hi,
> 
> In fact you should use
> \setupuserpagenumber[number=1]
> as in the following example:
> 
> %%%% begin example
> \starttext
> \startfrontmatter
> \setupuserpagenumber[numberconversion=Romannumerals]
> \input knuth.tex
> \stopfrontmatter
> 
> \startbodymatter
> \setupuserpagenumber[numberconversion=numbers]
> \setupuserpagenumber[number=1]
> \input ward.tex
> \stopbodymatter
> \stoptext
> %%%% end example
> 
> Best regards: OK
> 
> On 22 May 2014, at 11:37, Werner Hintze we.hintze@gmail.com wrote:
> 
> I don’t unterstand \startfrontmatter, \starbodymatter etc.
> 
> I thought, the frontmatter has roman numerals, the bodymatter normal and so on. This is not automatically so. Now founf how to make this, bus the problem is: I can’t reset the page number. I believed, it shoul work like this:
> 
> \startfrontmatter
> \setupuserpagenumber[numberconversion=Romannumerals]
> % some stuff
> \stopfrontmatter
> 
> \startbodymatter
> \setupuserpagenumber[numberconversion=numbers]
> \setnumber[page]{1}
> %some stuff
> \stopbodymatter
> This doesn’t work, may be because \setnumber is not a command in ConTeXt?
> 
> Where is my error?
> 
> If your question is of interest to others as well, please add an entry to the Wiki!
> 
> maillist : ntg-context@ntg.nl / http://www.ntg.nl/mailman/listinfo/ntg-context
> webpage : http://www.pragma-ade.nl / http://tex.aanhet.net
> archive : http://foundry.supelec.fr/projects/contextrev/
> wiki : http://contextgarden.net
> 
> If your question is of interest to others as well, please add an entry to the Wiki!
> 
> maillist : ntg-context@ntg.nl / http://www.ntg.nl/mailman/listinfo/ntg-context
> webpage : http://www.pragma-ade.nl / http://tex.aanhet.net
> archive : http://foundry.supelec.fr/projects/contextrev/
> wiki : http://contextgarden.net
> 
> ___________________________________________________________________________________
> If your question is of interest to others as well, please add an entry to the Wiki!
> 
> maillist : ntg-context@ntg.nl / http://www.ntg.nl/mailman/listinfo/ntg-context
> webpage  : http://www.pragma-ade.nl / http://tex.aanhet.net
> archive  : http://foundry.supelec.fr/projects/contextrev/
> wiki     : http://contextgarden.net
> ___________________________________________________________________________________