This DEV-only field was added in 836331ba5f69f2cc53b273ec073fae116db14302 by @sebmarkbage but it's not being used anymore to my knowledge.
current,
  workInProgress,
  primaryChildren,
  fallbackChildren,
  renderLanes,
) {
  const mode = workInProgress.mode;
  const primaryChildFragment = createFiberFromOffscreen(
    primaryChildren,
    mode,
    NoLanes,
    null,
  );
  const fallbackChildFragment = createFiberFromFragment(
    fallbackChildren,
    mode,
    renderLanes,
    null,
  );
  
  
  fallbackChildFragment.flags |= Placement;
  primaryChildFragment.return = workInProgress;
  fallbackChildFragment.return = workInProgress;
  primaryChildFragment.sibling = fallbackChildFragment;
  workInProgress.child = primaryChildFragment;
  if ((workInProgress.mode & ConcurrentMode) !== NoMode) {