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

The full json response I have put here https://gist.github.com/bdovh/e1d0a326964caad4108e378d23c0dcb1

and this is field describe extract only below

     {
        "aggregatable" : true,
        "aiPredictionField" : false,
        "autoNumber" : false,
        "byteLength" : 765,
        "calculated" : false,
        "calculatedFormula" : null,
        "cascadeDelete" : false,
        "caseSensitive" : false,
        "compoundFieldName" : null,
        "controllerName" : null,
        "createable" : false,
        "custom" : false,
        "defaultValue" : null,
        "defaultValueFormula" : null,
        "defaultedOnCreate" : false,
        "dependentPicklist" : false,
        "deprecatedAndHidden" : false,
        "digits" : 0,
        "displayLocationInDecimal" : false,
        "encrypted" : false,
        "externalId" : false,
        "extraTypeInfo" : null,
        "filterable" : true,
        "filteredLookupInfo" : null,
        "formulaTreatNullNumberAsZero" : false,
        "groupable" : true,
        "highScaleNumber" : false,
        "htmlFormatted" : false,
        "idLookup" : false,
        "inlineHelpText" : null,
        "label" : "Channel",
        "length" : 255,
        "mask" : null,
        "maskType" : null,
        "name" : "KbChannel",
        "nameField" : false,
        "namePointing" : false,
        "nillable" : false,
        "permissionable" : false,
        "picklistValues" : [ {
          "active" : true,
          "defaultValue" : false,
          "label" : "All Channels",
          "validFor" : null,
          "value" : "AllChannels"
        }, {
          "active" : true,
          "defaultValue" : false,
          "label" : "Internal App",
          "validFor" : null,
          "value" : "App"
        }, {
          "active" : true,
          "defaultValue" : false,
          "label" : "Public Knowledge Base",
          "validFor" : null,
          "value" : "Pkb"
        }, {
          "active" : true,
          "defaultValue" : false,
          "label" : "Customer",
          "validFor" : null,
          "value" : "Csp"
        }, {
          "active" : true,
          "defaultValue" : false,
          "label" : "Partner",
          "validFor" : null,
          "value" : "Prm"
        } ],
        "polymorphicForeignKey" : false,
        "precision" : 0,
        "queryByDistance" : false,
        "referenceTargetField" : null,
        "referenceTo" : [ ],
        "relationshipName" : null,
        "relationshipOrder" : null,
        "restrictedDelete" : false,
        "restrictedPicklist" : true,
        "scale" : 0,
        "searchPrefilterable" : false,
        "soapType" : "xsd:string",
        "sortable" : true,
        "type" : "picklist",
        "unique" : false,
        "updateable" : false,
        "writeRequiresMasterRead" : false
      }


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