https://docs.splunk.com/Documentation/Splunk/8.0.2/SearchReference/Eval The eval command calculates an expression and puts the resulting value into a search results field.
If the field name that you specify does not match a field in the output, a new field is added to the search results.
If the field name that you specify matches a field name that already exists in the search results, the results of the eval expression overwrite the values in that field.

Reference:https://docs.splunk.com/Documentation/Splunk/8.0.3/Knowledge/Usesearchmacros