If the code for $\text{ABC}$ is $\text{ZYX}$, what is the code for $\text{DEF}$?

The coding logic uses the reverse positional alphabet pairs (where the sum of positional ranks is always $27$):
* $\text{A }(1)⟷\text{ Z }(26)$
* $\text{B }(2)⟷\text{ Y }(25)$
* $\text{C }(3)⟷\text{ X }(24)$

Applying the exact same positional inverse matching rule to $\text{DEF}$:

* $\text{D }(4)⟷\text{ W }(23)$
* $\text{E }(5)⟷\text{ V }(22)$
* $\text{F }(6)⟷\text{ U }(21)$

Hence, the code for $\text{DEF}$ is $\text{WVU}$.