Our two installers are per-user (standard) and per-machine (admin). They contain the same executable code. The differences are on where the code and registry entries are installed to.
Default Install Location
The per-user install is installed by default under the user's AppData\Local folder (specifically, AppData\Local\Cirruspath, Inc.\Cirrus Insight for Outlook). The per-machine install is installed in Program Files by default (under Program Files\Cirruspath, Inc.\Cirrus Insight for Outlook). The install location is one of the reasons the per-machine install requires administrative privileges to install, and the per-user install does not.
Outlook add-ins are loaded via entries within the Windows Registry. The installers handle registering the add-in in the appropriate place to load on a per-user or per-machine basis. The per-user installer will place the registry keys under the HKEY_CURRENT_USER hive, while the per-machine installer places the entries under HKEY_LOCAL_MACHINE. This sets up Outlook to load the add-in on a per-user or per-machine basis.
Both the standard and admin installers are fairly standard MSI files. The difference between them is that the admin install needs to be run in an administrative context, or else it will not be able to write to the appropriate directories and registry keys. The included setup.exe is a bootstrap to handle this via right-click > Run As Administrator, and is described as the preferred install path on our webpage, but its not strictly necessary.
Both MSIs can be installed using msiexec.exe /i. The /a flag is not supported, so automated installations must be run in elevated privilege contexts (e.g. in an administratively-launch command line or, I'm hoping, somehow via SCCM).
One minor point: the admin install has a SLIGHT executable difference, in that the end user is NOT prompted for add-in updates, under the presumption they won't be able to update anyways. That means they'll be reliant on email notifications for new versions, or periodically checking the Settings dialog to see if a new version is available.