system-system-spacing in layout?

Hi,

NR 4.2.1 The \layout block
lists the paper-variables which may occur in \layout:
line-width, ragged-right, agged-last, indent, short-indent and system-count.

So why system-system-spacing has an effect here?

\layout { system-system-spacing.padding = 4000 }
{ R1 \break R }

Cheers,
  Harm