ยังหาไม่เจอว่า understood เป็น past tense หรือ past participle
Understood ในกรณีนี้มาจาก Is that understood?เป็น Passive Voice of Present Simple Tense
You can use understand in expressions like do you understand? or is that understood? after you have told someone what you want, to make sure that they have understood you and will obey you.
You do not criticize my grandchildren, do you understand?
I don't need it, understand?
I don't want to hear another word about it. Is that understood, Emma?
https://www.collinsdictionary.com/dictionary/english/do-you-understand-is-that-understood