EL Expressions
From Documentation
Many attributes of the configuration descriptor support EL expressions. It is the same as JSP EL and ZK EL expressions. Here we describe the implicit objects that are available to the configuration descriptor.
To invoke EL functions (i.e., static methods), please refer to the taglib, variable-resolver and xel-method elements.
If you're not familiar with EL expressions, please refer to ZUML Reference: EL Expressions.