Expressions are new in SSIS (meaning that they were not in DTS) but they allow you to create extremely flexible packages. Expressions can be used to assign values to variables, help you determine whether to execute a task, and to assign properties. If you ever found yourself writing ActiveX code (VBScript) in DTS to determine which task to execute based on a variable's value, then you probably can forego the scripting all together in favor of expressions! They are very powerful in SSIS and you'll want to have a solid understanding of how to write them and how to use them.
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