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Let's assume that you have a District table, a Contacts table and an Orders table. The District table is the master table to the Contacts table and the Contacts table is the master table to the Orders table.
You want to embed a report of all the orders in a district in the district record. Let's assume the key field in the District table is the Record ID#. But let's also assume we've renamed the Record ID# field in the Districts table to be "District ID#". Because we have a master-detail relationship between the Districts table and the Contacts table, there is a reference field in the Contacts table called something like "District ID# (ref)".
There is also a master-detail relationship between the Contacts table and the Orders table. Go to this relationship and add a lookup field that points to the "District ID# (ref)" field in the Contacts table.
Now you're ready to add a report link field to the District table. Call this report link field "Orders". Now set its source field property to "District ID#". Set the target table to Orders. Set the target field to "District ID# (ref)".
Now you can place this report link field on a custom form and you can choose to display it as an embedded report instead of a link. The embedded report will display all orders related to the district.