We can use Workbench tool to run Sobject Describe endpoint

https://workbench.developerforce.com/restExplorer.php?url=/services/data/v58.0/sobjects/SearchActivity/describe&autoExec=1

which provides information about five possible picklist values of this field.

[![enter image description here][1]][1]

Which include

 - All Channels(`AllChannels`) - assuming this is an aggregate option.
   
 - Internal App (`App`) - assuming this value corresponds to internal
   Knowledge search activity for users having Salesforce or Salesforce
   Platform licenses 

 - Public Knowledge Base (`Pkb`) - assuming this value
   corresponds to public site guest user profiles 

 - Customer(`Csp`) - assuming this value corresponds to the Experience Cloud Customer portal users which have Community and Community Plus licenses 

 - Partner (`Prm`) - assuming this value corresponds to the Experience Cloud
   Partner portal users which have Partner user licenses

  [1]: https://i.sstatic.net/shL2N.png