What is the "permissionTarget" parameter in GetEntityListWithPermissionMask() service?

I'm trying to determine a list of visible things for a particular user. Thanks for your help!

var tags = "vocabulary:tag";
var params = {
     permissionType: "PropertyRead" /* STRING */,
     maxItems: undefined /* NUMBER */,
     nameMask: undefined /* STRING */,
     permissionTarget: "William.Shatner" /* STRING */,
     type: "Thing" /* STRING */,
     tags: tags /* TAGS */
};
// result: INFOTABLE dataShape: RootEntityList
var result = Resources["EntityServices"].GetEntityListWithPermissionMask(params);