This article shows how to use a process parameter in a mapping expression.
The process
Here is the process, with the parameter named "fileName":
The mapping
In the following mapping, we use this parameter in the mapping expression of the "CITY" field, and as the "In File Name" parameter of the Load template :
Note that the mapping has to be wrapped in the process. It will not execute as a standalone mapping, because the parameter is defined in the process.