\definefontfeature
  [default:bold]
  [default]
  [axis={weight=500}]

\definefontfeature
  [default:heavy]
  [default]
  [axis={weight=800}]

\definefontfeature
  [default:light]
  [default]
  [axis={weight=250}]

\definefont
  [DemoFontBold]
  [file:NotoSansCJKsc-VF.otf*default:bold @ 10pt]

\definefont
  [DemoFontHeavy]
  [file:NotoSansCJKsc-VF.otf*default:heavy @ 10pt]

\definefont
  [DemoFontLight]
  [file:NotoSansCJKsc-VF.otf*default:light @ 10pt]

\startTEXpage
    \DemoFontBold  \char"5E2D\kern10pt\char"5E2E\kern10pt\char"5E2F\par
    \DemoFontHeavy \char"5E2D\kern10pt\char"5E2E\kern10pt\char"5E2F\par
    \DemoFontLight \char"5E2D\kern10pt\char"5E2E\kern10pt\char"5E2F
\stopTEXpage