How to api access relationship notes?
The Note is stored separately, so you can get it via chaining:

$result = civicrm_api3('Relationship', 'get', array(
  'sequential' =&gt; 1,
  'relationship_type_id' =&gt; 6,
  'api.Note.get' =&gt; array(
     'entity_table' =&gt; "civicrm_relationship",
     'entity_id' =&gt; "\$value.id",
  ),
));


To do that with the API explorer, use the Chain API Call button and enter the note parameters as JSON: {'entity_table':'civicrm_relationship', 'entity_id':'$value.id'}