Expression Operator
About
Thee Expression operator is used to write SQL expressions that define algorithms for one output parameter of the operator.
By default, the Expression operator contains one input group and one output group.
How to use it
Drag and drop it on the mapping
Choose the attribute for the input group
Open it to add the attribute for the output group
Select the output attribute
in the attributes properties (right panel), you can add a expression in the expression properties (see the picture below)
Reference
* Documentation