If you include a tag in your calculation formula for a field that doesn’t exist on the page, the calculation will normally return an empty value.
This means that if you have a formula like {field_123} + {field_456} you’ll only get a result if you have fields with the IDs 123 and 456 present on the page.
However, you might prefer the calculation to ignore a field if it’s not present and still return a value for those fields that are present. To enable this, go to WooCommerce > Settings > Product Add-Ons > Calculations and check the ‘Zero value for missing fields’ option.
The benefit of this option is that it allows you to create a single global calculation field which includes values for multiple fields, not all of which might be present on a single product.