Validation Property

Requirement : Action end date should always be greater than action start date.

Solution : BC Field level, set to the syntax of the validation property to

                        >=[Action Start Date]

Should throw an error if the field left empty

Requirement : The field "Date Of Birth" shouldn't be left empty by the user.

Solution: On BC level set the "Required" property to TRUE for the field.

Display Total in List Applet

Requirement : In the Opportunity List Applet, the requiement is to display the total Revenue made by all opportunities.
Solution : Siebel has an out of the box facility for such requirement.
Steps to fulfill this requiement.
In tools Goto Opportunity List Applet > List, make "Total Displayed" and "Total Required" property to TRUE.
Goto the Revenue listcolumn, "Total Required" property to TRUE.

Compile and see the result in the column.