SSIS - Expession Builder

It is an Editor, which creates expression using the variables and In-Built functions. Normally this editor can be reached for all elements in the package. the button with "..." [3 dots] is clicked to open this editor.
The property of the elements are set directly by assiging values to them, or by setting a expression and the expression is evaluvated in the runtime to set the values.
Almost every control's editor will have a Expressions tab, which can be selected, the tab contains a Field group named Expressions, by selecting the triple dot button again, will opens a Property Expression Editor. this contains two columns. first column contains drop down list, from which a field name can be selected and the second column contains a triple dot button, which actually opens a Expression Builder.
Expression Builder Editor contains 5 blocks.
1. Variable - list of variables available within the scope is listed.
2. Built in functions - list of functions are organized in groups and listed.
3. Description - displays a description of built in function, which is currently selected[Read only]
4. Expression - variables and functions can be dragged and droped here or expression can be manually keyed in.
5. Evaluvated Value - the value of the expression will be displayed, when the evaluvate expression is clicked, which throws exception if the expression is not set correctly.

The Built In Functions block will have type casting symbols and also the supported operators for evaluvating the expression.
once the expression is created succesfully the dialog can be closed, and the expression is assigned to the field. we can assign expressions to any no of fields.