The Set code allows you to set the value in the merged object.
Fld must be a field definition and value can be either a field definition or a constant value.
For example, [=[\UDF23]=[\System\Today]=] will set the User Defined Field 23 to today's date.
Note that the Set code can be used inside an If code to set a field to a value if a condition is met. Also, the set will not take place if there is an error in the merge process.