I found a bug in the network query class.
If I run following network query:
$query = array( 'tax_query' => array( array( 'taxonomy' => 'category', 'terms' => array( 'general' ), 'field' => 'slug', 'operator' => 'IN' ) ) );
Then I get all the posts that are saved to the General category AND also posts that have same object_id than general category has in network term relationships table.
In other words, if General category has object id 275 in the network term relationships table, the results include also the post, which has the object id 275 in the network posts table.
I hope that the author of the plugin will be informed about this bug.