The Set code allows you to set the value in the merged object.

Syntax: [=<fld>=<value>=]

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.