What are calculated fields based on?

Calculated fields in Splunk are based on eval expressions, which are used to transform or manipulate field values at search time. This allows users to create new fields or modify existing fields based on calculations, logical operations, or string manipulations. Eval expressions provide a rich set of functions to perform operations on the data, such as mathematical computations, conditional statements, and string formatting.

While it is true that keyword searches and stats commands can involve the use of fields and may impact the results or context in which calculated fields are applied, they do not serve as the basis for defining what a calculated field is. The primary mechanism for creating calculated fields is indeed the use of eval expressions, making this option the correct answer.