The ref report dialog does a reverse lookup of all the references on a specific block.

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The first dialog allows you to choose the block to do the reverse reference lookup for. Alternatively, you could browse to the target block in the BBT, expand it, and double click on the "Ref Count" field.
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This is the reference report. At the top of the dialog is displayed the expected ref count (from the BBT) and the discovered ref count. The discovered ref count is calculated by walking through the entire NBT and BBT looking for and tracking outgoing references to the target ID.

Below that is a table view giving the results of the search.