In Splunk, the NOT operator is used to exclude events from your search results. The search index=network NOT StatusCode=200 will return all events in the 'network' index where the StatusCode is not 200. This includes events where the StatusCode field is present and has a value other than 200, as well as events where the StatusCode field is not present at all.
References:The use of the NOT operator in SPL (Search Processing Language) is consistent with the information provided in the Splunk documentation and resources, which describe how to generate efficient searches and make the most of Splunk's capabilities