The use case is that you create a custom table for storing financial/budgeting type data for a project. You add a number custom field for "Planned Spend" and another number custom field for "Actual Spend". The desire is to have another calculated field named "Spend Delta" that would be derived based on a calculation of "Planned Spend" - "Actual Spend" which would be specified as part of the "Calculation" type properties. Similarly, you might want to have a derived custom field that displays the % of Budget Spent so the custom field properties would allow you to specify the equation is "Actual Spend"/"Planned Spend".
I used a custom table example but it should be available as a general custom field option. But, it seems especially compelling to be able to perform operations between different fields in a custom table.
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