The prefixes are described here.
But by default pandoc treats the top level as a section and not a chapter, as discussed in the caveats:
Because pandoc-crossref offloads all numbering to LaTeX if it can, chapters: true has no direct effect on LaTeX output. You have to specify Pandoc's --top-level-division=chapter option, which should hopefully configure LaTeX appropriately.
The prefixes are described here.
But by default pandoc treats the top level as a section and not a chapter, as discussed in the caveats: