If someone tells you an instruction/order, can you answer with 'Understood', rather than 'I understand'? I felt like I heard that being used, but I'm not so sure.
"Be sure to clean up after yourself."
In this exchange, understood is not being used as a short form of I understand. Instead, it's being used as a short form of something else.
"✘ (I) underst(and)."
"✔ (It is) understood."
"✔ (I have) understood."
The full version of the sentence is assumed to exist, even though it's not actually being spoken. Either of the full versions will work in the context.
Similarly, I hear kitchen staff shouting "Heard!" in response to head chefs reciting orders. (At least that's what I've heard on cooking shows on TV.)