The Most Recent Search Engines report and any report using search phrases and keywords is controlled by the list of search engines in the keywords.ini file. This file not only lists the known domains of search engines, but also the query parameter value they use to define a search keyword.
The KeywordIndicator shows the query parameter name used by the Search Engine to specify the search phrase used. Since search engines perform other activities in addition to searches, there may be referring traffic from a search engine that lacks the KeywordIndicator value. Without the KeywordIndicator, the search engine is treated as a referrer, and not a search engine.
Example of a referrer:
Example of a search engine:
As a result, the visit traffic in Search Engines-related reports may not correlate with the visit traffic in the Referring Domains report.