In the Visual Studio designer you can right click on a SSIS package and designate it as an "Entry Point" package"
Doing a search I found this page on MSDN which states:
The value of 1 signifies that the package is meant to be started directly. The value of 0 signifies that the package is meant to be started by another package with the Execute Package task. The default value is 1.
With this flag enabled and disabled I have been able to execute a package directly.
What is the purpose of enabling or disabling this flag? Is it merely to document the intentions of your own SSIS packages or does SQL Server/SSIS behave differently when it enabled or disabled?