A custom field worksheet will provide for a method of calculating the owning field’s value from a set of detailed entries that are specific to the owning field.
A custom field worksheet is one that is available only to a specific field in a specific form module.
A simplified example of a possible custom field worksheet would be one that defines the value of a stock asset. The calculation worksheet could have detailed fields for the Unit Value and the Number of Shares, and then the result would be the multiplication of these two fields. This result would be carried to the Amount column of this stock item on Form 706’s Schedule B. There would be a corresponding description field in this item/row that describes the stock.