When you query your entities on values on a field, for example you want to get all nodes via a node reference field. In our case we have an event and we want to know all referenced sessions to it.
If you want to render only one field of your entities you can in drupal 7. Here in the example we have an entity called model which contains an image field.
To render the field you have to specify to the field_attach_view function which entity type, the entity and the view mode you want to use. The function returns a renderable array.
If you want to save only a specific field on your entity instead of saving the whole entity you can. In a submodule of the fields module you ll find a module called field_sql_storage module. In that module all functions available for the sql storage backend can be used. One of them is field_sql_storage_field_storage_write, which you give your entity, type, operator (update, insert) and which fields to save.
