Which of the following is NOT a command used for data aggregation in Splunk?

The command that is NOT used for data aggregation in Splunk is associated with the option that pertains to notable events. In Splunk, the 'stats', 'chart', and 'timechart' commands are explicitly designed for aggregating data.

The 'stats' command allows users to perform statistical calculations on their data, like calculating averages, counts, sums, etc. This makes it a vital tool for aggregating data based on various fields.

The 'chart' command transforms data into a table format that can be visualized as different types of charts. This is another method to aggregate and display data visually.

The 'timechart' command is specifically focused on time-series data aggregation, enabling users to plot data over time with various statistical calculations.

On the other hand, the option that focuses on 'notables' relates to a feature used to track significant events in a security context. It is part of the incident response processes rather than a command used for data aggregation. Therefore, that choice does not fit with the others, making it clear why it is the correct answer in this context.