In any of these installation transactions, viz. initial installation, repair, reinstallation, on-demand installation or patching an MSI, The REINSTALLMODE has an affect on it.
Here is what "amus" means:
- a - Force all files to be reinstalled, regardless of version
- m - Rewrite all registry keys that go to HKEY_LOCAL_MACHINE or HKEY_CLASSES_ROOT
- u - Rewrite all registry keys that go to HKEY_CURRENT_USER or HKEY_USERS
- s - Reinstall shortcuts and icons
The mentioning of "a" in amus forces the reinstallation of files on your machine irrespective of the file version rules. Every file updated by REINSTALL property will be updated in this case.
The sequence of events which happen actually depend on the authoring of your patch.
If your patch contains the full file, the installer will not access the source to reinstall that file, but if your file is the delta of the file, like an update to say ini file, then the patch will access first the machine file and update it, and if machine file is not accessible to the patch, then in that case, the installer will go and grab the file from the original source location and will work from there.
REINSTALLMODE=amus is not the recommended solution, but sometimes situations become inevitable and we need to use them.