Question about scale information in Global Bundle Adjustment

I have some doubts when using openSfM for global BA, BA is a 7-dofs problem with 3 axes, 3 rotations scale. If the Pose of some of the images is not fixed in BA only through images, the scale of the whole scene may change after BA. So similar to colmap, it fixes the Pose of the first camera and the translation x component between the two cameras, which will introduce scale information. I didn't find out which parts of the global BA are fixed by the rig instance model of openSfM, There is actually a function "SetGaugeFixShots" in bundle_adjuster.cc and AddTranslationPriorError into problem, but I found that it was not used in the reconstruction. I hope you can give me some inspiration。

void BundleAdjuster::SetGaugeFixShots(const std::string &shot_origin,
                                      const std::string &shot_scale) {
  Shot *shot = &shots_.at(shot_origin);
  shot->GetRigInstance()->SetParametersToOptimize({});
  gauge_fix_shots_.SetValue(std::make_pair(shot_origin, shot_scale));
}
// Gauge fix
if (gauge_fix_shots_.HasValue()) {
  const auto &gauge_shots = gauge_fix_shots_.Value();
  auto instance1 = shots_.at(gauge_shots.first).GetRigInstance();
  auto instance2 = shots_.at(gauge_shots.second).GetRigInstance();
  const double norm =
      (instance1->GetValue().GetOrigin() - instance2->GetValue().GetOrigin())
          .norm();

  ceres::CostFunction *cost_function =
      new ceres::AutoDiffCostFunction<TranslationPriorError, 1, 6, 6>(
          new TranslationPriorError(norm));

  problem.AddResidualBlock(cost_function, nullptr,
                           instance1->GetValueData().data(),
                           instance2->GetValueData().data());
}

Start by tracing the reconstruction path that configures global bundle adjustment and inspect bundle_adjuster.cc, especially SetGaugeFixShots and AddTranslationPriorError. Compare those call paths with the rig instance model and determine whether gauge and scale constraints are applied. Done means documenting the current scale behavior and identifying the concrete change needed, if any.