There are 3 types of properties in Windows Installer; namely: Public Property, Private Property and Restricted Public Property.
Public property can be changed from command line while installing the package whereas private properties cannot be changed from command line. Restricted public properties can be only changed by a system administrator or by a user who has elevated privileges. To include public properties in Restricted public properties, add them to the SecureCustomProperties property.
Generally we add INSTALLDIR as Restricted public property.