Imagine you have created an order entry app in Knack that has an invoice HEADER table (that contains the CUSTOMER field) and an invoice DETAIL file (that contains the PRODUCT ID field). An immediate challenge within Knack is that the pricing from the ITEM table has to be added with Submit Rules after a record is added to DETAIL. But that I can live with.
Imagine now the scenario where this is a PRICING table that contains the bespoke prices offered for each item to each customer. And now try to imagine updating the price in DETAIL with form rules. You can't. And that's the problem.
To rectify this situation, the list of available tables needs to include the PRICING table but, at present, it does not.