This property contains an integer value (1 to 32,767) that determines which Windows Installer features are to be installed by default (any feature with a "Level" value which is less than or equal to the value of this property). Other features will need to be selected for install by the user.
Other properties may override the processing of this variable. Please see Microsoft's online documentation for the "INSTALLLEVEL" property (or "MSI.CHM") for more information.
The "Choose Setup Type" dialog (in "uisample.msi") allows the user to choose between "Typical", "Custom" and "Complete", these sorts of dialogs will modify this property. You could of course redesign the dialogs to show more choices but you would typically keep these due to the difficulty of dialog design (if for no other reason).
To choose the correct values for a feature you would need to know what value is set for "Typical" and "Complete" (in "uisample" the values are 3 and 1000 - as seen in its "ControlEvent" table). A verbose install will help if you are unsure.