在训练 XGboost 模型时，打印的一些信息与“额外节点”有关。我在文档中的任何地方都找不到对这些的解释。什么是额外节点？

[14:13:09] C:\dev\libs\xgboost\src\tree\updater_prune.cc:74: tree pruning end, 1 roots, 54 extra nodes, 0 pruned nodes, max_depth=5
[14:13:09] C:\dev\libs\xgboost\src\tree\updater_prune.cc:74: tree pruning end, 1 roots, 58 extra nodes, 0 pruned nodes, max_depth=5
[14:13:09] C:\dev\libs\xgboost\src\tree\updater_prune.cc:74: tree pruning end, 1 roots, 48 extra nodes, 0 pruned nodes, max_depth=5
[14:13:09] C:\dev\libs\xgboost\src\tree\updater_prune.cc:74: tree pruning end, 1 roots, 46 extra nodes, 0 pruned nodes, max_depth=5
[14:13:10] C:\dev\libs\xgboost\src\tree\updater_prune.cc:74: tree pruning end, 1 roots, 48 extra nodes, 0 pruned nodes, max_depth=5
[14:13:10] C:\dev\libs\xgboost\src\tree\updater_prune.cc:74: tree pruning end, 1 roots, 50 extra nodes, 0 pruned nodes, max_depth=5
[14:13:10] C:\dev\libs\xgboost\src\tree\updater_prune.cc:74: tree pruning end, 1 roots, 60 extra nodes, 0 pruned nodes, max_depth=5
[14:13:10] C:\dev\libs\xgboost\src\tree\updater_prune.cc:74: tree pruning end, 1 roots, 44 extra nodes, 0 pruned nodes, max_depth=5
[14:13:10] C:\dev\libs\xgboost\src\tree\updater_prune.cc:74: tree pruning end, 1 roots, 50 extra nodes, 0 pruned nodes, max_depth=5
[14:13:11] C:\dev\libs\xgboost\src\tree\updater_prune.cc:74: tree pruning end, 1 roots, 46 extra nodes, 0 pruned nodes, max_depth=5