Lat's say you want to show the results of your own document content type, but when you search all possible options show up. What you can do then is to add a rule to your search scope.

Go to site settings > Scopes > Click on your custom scope

The click on the 'add rule' option and in the next screen choose the 'Property Query' option. Then choose the 'contentclass' option in the 'Property Query' section. As a value for this property you must fill out the value STS_ListItem_<your list template ID>. For example if you used the template ID 10150 in your list feature, the contenclass must be equal to STS_ListItem_10150. The behaviour option must be set to Require.

After recrawling, the search result is narrowed to only your list items.