Metonyms are somewhat related to metaphors;
they are the usage of an object in a sentence instead of something of that object.
For example, in the sentence "Orders came in from the office", even though "office" really stands for "boss" (or something), "office" is used because it has some relation to "boss".
If we were to translate this sentence to Lojban with the metonym usage, we'd get
{ lo briju cu minde }.
Because Lojban is a logical language to some extent, it's incorrect to say that -- offices don't give orders.
Just like { mi cizra }, which needs a {jai} (because I'm not an event), { lo briju cu minde } should also be fixed with a {jai}-like cmavo, X. X, contrary to {jai}, would be in SE (because we don't need to use the {jai}-plus-sumtcita feature).
In order to fix the example, we'd say {zo'ei lo briju cu minde} (which is not very natural), or, using X, {lo briju cu X minde}.
Explaining it with a simple relation might help:
tu'a : jai : (kai'a) ::
zoi'e : X : Y
It's also worth considering the Y, because it could be useful.
~ mi'e la uakci mu'o