Why multiple MSIExec.exe processes run during an installation?

A number of MSIExec processes can be running during an installation. The reason for this is that Windows Installer uses a client-server model for performing installations. Additionally for security reasons, Windows Installer hosts DLL and script custom actions in a “sandbox” process. Depending on how the install was initiated, one of the MSIExec processes can be the client process. Another MSIExec process is Windows Installer service. Any remaining MSIExec processes are usually sandbox processes for hosting custom actions. The determination as to which MSIExec process will serve as the sandbox process for a script or DLL custom action depends in part on whether the custom action will run elevated or impersonated and whether the custom action is 32-bit or 64-bit.