What is a "leaf expression"? What is a &quot;leaf expression&quot;?

Partition-clause expressions are limited in the following ways:
....

You can specify leaf expressions, functions, and operators in the partition clause expression.
All leaf expressions in the partition clause must be either constants or columns of the table.


Source https://my.vertica.com/docs/6.1.x/HTML/index.htm#18125.htm
I'm pretty sure leaf expression means something like "a?b:c". A discrete value from a conditional.

But basically the overall message is saying you need to avoid null values. They can't partition on a null.