Which option allows you to modify existing fields or create new ones in the Multi-Row Formula tool?

The ability to modify existing fields or create new ones in the Multi-Row Formula tool is facilitated through the update existing field feature. This option allows users to select a specific field and apply a formula that alters its values across multiple rows. In essence, when using this selection, you can dynamically change the contents of an existing field based on calculations, conditions, or aggregations from neighboring rows.

This capability is crucial for data transformation tasks where cumulative calculations or inter-row references are required. By updating existing fields, you can streamline your workflow and maintain clarity, as the changes are reflected directly within the original dataset rather than generating new fields that could complicate subsequent analysis processes.

In contrast, the other options serve different purposes within the Multi-Row Formula tool. Building an expression is essential for specifying the actual formula, but it does not inherently modify fields. The "Group By Field" option is primarily used for aggregating data rather than editing it. Lastly, configuring null values pertains to how the tool treats null or missing data rather than how fields are updated or created. Understanding these distinctions reinforces the utility of updating existing fields specifically when using the Multi-Row Formula tool.