Wolfgang Schuster mailto:schuster.wolfgang@gmail.com 10. Oktober 2016 um 10:39 prefixconnector is the separator between the section number (=prefix) and the float number (=figure number), the symbol at the end of the float number is called numberstopper. In your example you don’t have a section which can act as prefix number no did you change which prefixes should be shown (by default only chapters are used as prefix).
The suffix key you use (together with suffixconnector and suffixstopper) is used to show the number for subfigures, e.g. figure 3.a.
Here is a working example for the use of the prefixconnector key.
\starttext
\section{This is a section}
\setupcaptions[prefixsegments=section,prefixconnector=-]
\startplacefigure[title=\tex{setupcaptions[prefixconnector=-]}] \externalfigure[dummy][width=.5\textwidth] \stopplacefigure
\stoptext
Wolfgang