Symantec WINDOWS INSTALLER EDITOR 8.0 Reference page 492

Windows Installer Editor Reference
Note
User context and system context are relevant only on locked-down computers.
Actions run in system context are run with elevated privileges by the Windows
Installer service. Actions run in user context are run with the current user's
privileges.
Processing
The main installation thread can control the custom action thread in different ways.
Options in this drop-down list determine how the custom action thread is controlled. See
Custom Action Return Processing Options and Synchronous and Asynchronous Custom
Actions in the Windows Installer SDK Help.
The Processing options are not available if the custom action type is Set Property or Set
Directory and are limited to synchronous for Install MSI custom actions.
Synchronous
Run the custom action synchronously to the main installation thread. Windows
Installer waits for the custom action to complete before continuing the main
installation. The exit code of the custom action must be 0 to indicate success. Use
this method for Windows Installer to wait for the success of the action before
continuing.
Synchronous, Ignore exit code
Run the custom action synchronously to the main installation thread. Windows
Installer waits for the custom action to complete before continuing the main
installation. Use this option if success of the action is unnecessary to continue with
the installation. The exit code of the custom action is ignored.
Asynch, Wait at end of sequence
Run the custom action asynchronously to the main installation thread. Windows
Installer runs the custom action simultaneously with the main installation. At the
end of the script, Windows Installer waits for the exit code from the custom action
before continuing. Use this if the installation is not dependent on completion of this
action, but you want to check the exit code. This option is not available for Install
MSI custom actions or if you selected Rollback Only in the In-Script Options list
above.
Asynch, No wait
Run the custom action asynchronously to the main installation thread. This means
that Windows Installer runs the custom action simultaneously with the main
installation. Windows Installer does not wait for completion of the custom action and
does not check the exit code. This option is not supported for Install MSI custom
actions. This option is not supported for Install MSI custom actions or if you selected
Rollback Only in the In-Script Options list above.
Scheduling Options
If you add the custom action to both the UI Sequence and the Execute Sequence, but
you want to limit the number of times it actually runs, select an option here. See Custom
Action Execution Scheduling Options in the Windows Installer SDK Help.
Always Execute
Select this to have the action execute in all sequences that you added it to.
Run first time
Select this to have the custom action execute only the first time Windows Installer
encounters it.
Custom Action Reference
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