Which of the following attributes is NOT required when defining a transform?

When defining a transform in Splunk, certain attributes are essential for the system to process data correctly according to the defined rules. Among the options, SOURCE_KEY is not a required attribute.

The REGEX attribute is necessary because it specifies the regular expression that the data must match for the transformation to be applied. Without this regex, the system wouldn't know what part of the data to act upon.

The DEST_KEY attribute is also mandatory; it indicates where the transformed data should be sent or how it should be stored in the event's fields. This key is crucial for directing the output of the transformation process.

The FORMAT attribute is needed to define how the resulting data should be formatted, enabling the accurate representation of the transformed data.

In contrast, SOURCE_KEY is optional as it can be used to refer to specific fields in the incoming data, but it is not a strict requirement for every transform definition. This flexibility allows users to define transformations without having to specify a source key, simplifying the process in cases where the default behavior is sufficient.