SATB vocal score example

[Johan Vromans]

Hi,

In section D.4.1 of the manual for 2.8.7, there's nice example of a
SATB score.

In the example, the keyword \global is used in a way I cannot deduce
from the docs. In short (please see the doc section for the full
example):

  sopMusic  = \relative c'' { ... }
  altoMusic = \relative c'' { ... }
  ...
  \score {
    \new ChoirStaff <<
      ...
      \new Staff = women <<
        \new Voice = "sopranos" { \voiceOne << \global \sopMusic  >> }
        \new Voice = "altos"    { \voiceTwo << \global \altoMusic >> }
      >>
      ...
  }

What is the function of \global in \voiceOne and \voiceTwo?
Leaving it out does not seem to make a difference.

Just curious.

-- Johan