Check for Terminal Server in WiX - installation

Question

Recently I had to "pre-select" an feature inside an WiX product. If the target machine is an Terminal-Server, the Feature should be selected for install.
To check, if the machine realy is used as Terminal-Server or not isn't required, since the installing user can still deselect the feature later.
One way to check for Terminal-Server (and not just an machine with RDP enabled) is to use GetVersionEx-API to check the wSuiteMask.
How to check that in WiX?

Answer 1

First we had to link WixUtilExtension.dll (Project-References->Add Reference)
Then use INSTALLLEVEL (as per mrnx's advice) to switch between different Features. We default it to "2" (All below 2 will be selected in feature-selection-screen, all above 2 will be de-selected):
<!--Default Features: All with levl 1-->
<Property Id="INSTALLLEVEL" Value="2" />
Use feature-selection-UI-template (Maybe you have to reference WixUiExtension.dll)
<UI>
<UIRef Id="WixUI_FeatureTree" />
</UI>
Now reference the properties, which we want to know:
<!--Terminal-Server?-->
<PropertyRef Id="WIX_SUITE_TERMINAL" />
<!--On Terminal-Server this property will be null, if multiple RDP-sessions are allowed-->
<PropertyRef Id="WIX_SUITE_SINGLEUSERTS" />
And finally in our Feature-list we have to use the properties inside a condition. The Feature should be de-selected per default, so we set it to one Level higher than the default "2" --> "3".
<Feature Id="TerminalServerServiceFeature" Level="3"
Title="(Feature-Title, i.E.)Terminal-Server-Service"
AllowAdvertise='no'
InstallDefault='local'
Absent='allow'
Description="Some description for the user, when he clicks on the feature" Display="expand"
ConfigurableDirectory='APPLICATIONFOLDER'
>
<!--Reference to the component (or componentgroup) we want to install-->
<ComponentRef Id="TerminalServerServiceComponent" />
<!--This sets this feature to installlevel 1, if the condition evals to true-->
<Condition Level="1">(WIX_SUITE_TERMINAL="1") AND NOT (WIX_SUITE_SINGLEUSERTS)</Condition>
</Feature>
Testing:
To check, why the condition fails, you have to check the installer log (msiexec /i YourMsi.msi /l*v "log.log").
Something along the lines
MSI (c) (8C!98) [11:18:27:663]: PROPERTY CHANGE: Adding
WIX_SUITE_SINGLEUSERTS property. Its value is '1'.
for a machine with RDP but without multi-rdp-session-support and
MSI (c) (4C!E0) [11:26:15:422]: PROPERTY CHANGE: Adding
WIX_SUITE_TERMINAL property. Its value is '1'.
for a machine with RDP and multi-rdp-session-support.