以下英文句子中的 understood 的主语是谁？ 从语法角度怎么解释？ 谢谢啦！

Question

This assessment did not include factors known to contribute to ice flows but not understood well enough to estimate with confidence.

Answer 1

This assessment did not include factors known to contribute to ice flows but not understood well enough to estimate with confidence.
这个评估不包括已知因素导致冰流，但不能够（令人）很好理解，程度导致估计的信心。
这样翻译，意义就明了。understood 的主语，也能够明确。追问

？？？

Answer 2

This assessment did not include factors //known to// contribute to ice flows but not understood well enough to// estimate with confidence.

这一估测并未考虑到那些影响冰流的人类已知的但尚未理解透彻的因素，只有对这些因素足够了解才能准确做出估测。

understood相当于是known的替代表达，可以简单看成是factors known to but not known well enough to...

主语就是factors