Explanation/Reference:
Explanation:
Property expression is an expression that is assigned to a property to enable dynamic update of the property at run time. For example, a property expression can update the To line that a Send Mail task uses by inserting an e-mail address that is stored in a variable.
An expression can be added to a package, task, Foreach Loop, For Loop, Sequence, Foreach enumerator, event handler, a package or project level connection manager, or log provider. Any property of these objects that is read/write can implement a property expression. Integration Services also supports the use of property expressions in some custom properties of data flow components. Variables and precedence constraints do not support property expressions, but they include special properties in which you can use expressions.
Property expressions can be updated in different ways:
User-defined variables can be included in package configurations and then updated when the package is deployed. At run time, the property expression is evaluated using the updated variable value.
System variables that are included in expressions are updated at run time, which changes the results of the property evaluation.
Date and time functions are evaluated at run time and provide the updated values to property expressions.
Variables in expressions can be updated by the scripts that the Script task and the Script component run.
References: http://msdn.microsoft.com/en-us/library/ms141214.aspx
http://blog.sqltechie.com/2012/11/expression-task-in-ssis-2012.html