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?