What does "&" and "!" Mean before the property name?

Question

I am reading a WIX script written by others. There is some code that confuses me.

    <Custom Action='UnLoadSchedulerPerfCounters' After='InstallInitialize'><![CDATA[(Installed) AND (!Scheduler = 3)]]></Custom>

    <Custom Action='RollbackSchedulerPerfCounters' After='WriteRegistryValues'><![CDATA[(&Scheduler = 3)]]></Custom>

So what is the difference between the "Planner" and the "Planner"? Is there any special meaning when a property is prefixed with "&" or "!"?

Answer 1

From http://www.tramontana.co.hu/wix/lesson5.php#5.3 :

Providing some special characters names will give them extra meaning that:

%     environment variable (name is case insensitive)
$     action state of component
?     installed state of component
&     action state of feature
!     installed state of feature

The last four can return the following integer values:

-1   no action to be taken
1    advertised (only for components)
2    not present
3    on the local computer
4    run from the source

Answer 2

Windows. . MSI SDK: http://msdn.microsoft.com/en-us/library/aa368012.aspx.