What should I use as Cognito's UserContextData => EncodedData?

In all AWS Cognito SDKs in most functions you can pass an UserContextData parameter to feed Cognito's Advanced Security feature:

$result = $client->forgotPassword([
  'AnalyticsMetadata' => [
    'AnalyticsEndpointId' => '<string>',
  ],
  'ClientId' => '<string>', // REQUIRED
  'SecretHash' => '<string>',
  'UserContextData' => [  // <=================== THIS
    'EncodedData' => '<string>',
  ],
  'Username' => '<string>', // REQUIRED
]);  

This field expects some EncodedData.

What should I put in UserContextData and how do I "encode" it?

When using an Admin* function like AdminInitiateAuth I can send unencoded fingerprinting data through ContextData:

$result = $client->adminInitiateAuth([
    [...]
    'ContextData' => [
        'EncodedData' => '<string>',
        'HttpHeaders' => [ // REQUIRED
            [
                'headerName' => '<string>',
                'headerValue' => '<string>',
            ],
            // ...
        ],
        'IpAddress' => '<string>', // REQUIRED
        'ServerName' => '<string>', // REQUIRED
        'ServerPath' => '<string>', // REQUIRED
    ],
    [...]
]);

The documentation does not help: