[a_seattle_ian]

That it came from Donald Rumsfeld in the context of what we know now and what he surely knew then is why it's such a good quote. The words basically say nothing but are also true about everything. So it can implicit be a warning that there is probably some bullshit going on or someone has a sense of humor and is also warning people while also avoiding the subject - of course just my opinion. How people actually use it will depend what the audience agrees it to mean.

[KnobbleMcKnees]

The common use I'm referring to is similar to the OP, which is using it as a framework for assessing risk. In particular, aligning a team on the "known unknowns" is critical to building the confidence and alignment needed as a group to be able to deal with unquantifiable/inestimable risk.

[a_seattle_ian]

I just took a look at that wiki article for Rumsfeld's usage of the "There are unknown unknowns" and I had no idea that he barrowed this phrase to frame his arguments and I was only familiar with that context, unfortunately.