How to make a custom WDK qualifier

WDK features, definitions and settings can be scoped so they are presented only when the user's context or environment matches the scope definition.
In other words you have a filtering mechanism using qualifiers. WDK provides the following standard qualifiers:
- DocbaseNameQualifier (scope: docbase, name of the docbase to which application connects)
- DocbaseTypeQualifier (scope: type, matches the document type)
- PrivilegeQualifier (scope: privilege, matches user privileges)
- RoleQualifier (scope: role, matches user role)
- ClientEnvQualifier (scope: clientenv, values: "webbrowser", "portal", "appintg", or "not appintg")
- AppQualifier (scope:application, matches the application name)
- VersionQualifier (scope: version, mathces the application version)
- EntitlementQualifier (scope:entitlement, checks entitlement evaluation classes)
- ApplicationLocationQualifier (scope:location, matches the navigation location)

However you might need to use custom scoping, so you must create a custom qualifier.
To create a cusom qualifier, you must perform the following steps:
1) Create the custom qualifier class, which implements the IQualifier interface. Here's a sample:
public class CustomQualifier implements IQualifier {

final static public String QUALIFIER_NAME = "customQualifier";

public String[] getAliasScopeValues(String strScopeValue) {
return null;
}
public String[] getContextNames() {
return new String[] { QUALIFIER_NAME };
}
public String getParentScopeValue(String strScopeValue) {
return null;
}
public String getScopeName() {
return QUALIFIER_NAME;
}
public String getScopeValue(QualifierContext context) {
String sCustomQualifier = "";

// custom code here to find the qualifier value to be set
// this code is called often, so it must not be 'heavy'
// consider using the cache

return sCustomQualifier;
}

2) Add your qualifier definition to app.xml file of your custom layer (usually custom folder):

inside <qualifiers> tag add your qualifier's class full name:
<qualifier>com.mycompany.wdk.qualifier.CustomQualifier</qualifier>

3) Use custom scoping & filtering in your components' xml definitions:
a)
<scope customQualifier="someValue">
....your definitions here...
</scope>

b)
<filter customQualifier="someValue">
    ....your definitions here...
</filter>

That's all. Keep in mind that qualifiers impact application performance because the qualifier's class is called on each read of definitions. So try to avoid adding custom qualifiers if you have other options.