The K-attribute in the Topic and TopicEx tags is for keywords related to the topic. The keywords will be listed in the help file index, and they are also used when linking to related topics. Note that there is a bug in WinHelp regarding this; when clicking the Related Topics button, only the first keyword listed in the K-attribute for the selected topic will be searched for among the keywords in the K-attribute for the other topics.
The keywords must be separated with ';'. Sub-entries (one level) in the help file index can be created by using ',' as below:
|<TopicEx K="keyword1; keyword2; keyword, subkeyword">|
<Title>Topic 1 title</Title>
Topic 1 Body