Here are the steps to do this:
1. Create a content scope in the FAST Content SSA. I have created a content source with the name FASTContentSource.
2. Now we need to run the following PowerShell command:
New-SPEnterpriseSearchQueryScope -SearchApplication "FAST Search Query" FASTDemoScope -Description "Content Source Demo Scope" -DisplayInAdminUI 1 -ExtendedSearchFilter contentsource:FASTContentSource
Here Search Application should be the FAST Query SSA. The DisplayInAdminUI determines whether this scope is visible in the Central Administration. The ExtendedSearchFilter create the FQL Filter which determines which rule needs to be assoicated with this scope. In my case I am creating a filter based on a content source.
3. Once this is done a scope will be created in the FAST Query SSA.
4. We need to wait for the scope to be compiled or we can start manually the compilation.
5. The strange thing is even after compilation on the UI the scope will not show any rules or any Items. The number of items would be empty as well rules will be empty.
6. Once this is done the scope is ready to be queried. To scope against this query we need to use the following syntax in the search box:
We can also create the FQL filtering on the existing scopes by using the following query:
$DemoScope = Get-SPEnterpriseSearchQueryScope -SearchApplication "FAST Query SSA" -Identity FASTDemoScope
$Demoscope.Filter = 'contentsource:"FASTContentSource"'
Same way we can also set up other FQL filters. Suppose we want to see only results having some specific Author we can set a filter like
$DemoScope.Filter = "Author:MyAuthor"