After ExecuteAction has fired in InstallUISequence, the installation continues into InstallExecuteSequence. During this phase, changes are made to the computer such as laying down files, updating the registry, and adding a new entry in Programs and Features. This part of the installation is called the "server side" and the InstallUISequence table is called the "client side", which is a way of conceptualizing that the two are run in different sessions and with different privileges. The client side runs as the user who launched the MSI while the server side is run as the LocalSystem user.

If you install with logging turned on you can see the split between the client and server. Actions that occur during the first half start ...

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