Continuing from my last post on variable basics ,you can click here to view MSBI # 10 - SSIS # 3 - Familiar with SSIS Variables from 0 to Expert (Part - I - Introduction) - Vishal Pawar's Blog

And then on for System Variable MSBI # 11 SSIS # 4 Familiar with SSIS Variables from 0 to Expert (Part II - System Variable) - Vishal Pawar's Blog

Where SSIS Variables are used

• Variables Usage Scenarios
• Property Expressions
• Data Flow Expressions
• Precedence Constraint Expressions
• Parameters and Return Codes
• For Loop Expressions
• Parent Package Variable Configurations
• Script Task and Script Component

For more Details on above usage Click Here

How to: Set the Properties of a Variable

• The Variables window.

• The Properties window. The Properties window lists properties for configuring variables that are not available in the Variables window: Description, EvaluateAsExpression, Expression, ReadOnly, and ValueType.

  For more Details on above usage Click Here

If you use Scripting tasks, the CLR or build components for SSIS, you will need to convert values from their SSIS types into .Net types. Here's a quick list of the SSIS data types and their .Net companion.

For more Details on SSIS variable you can visit following link

• http://www.techrepublic.com/blog/datacenter/using-user-defined-variables-in-sql-server-2005-integration-services-packages/237
• Using System Variables as Parameters in an OLE DB Source Query
• SSIS Package - User Variables - Robert Robbins 

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Link Resource # 11 August 1 – August 2 « Dactylonomy of Web Resource

Hope this helps !!