It's a hook used from the Features module. Truly, the feature.api.php file, which should contain a description of all the hooks used by a module, contains a description for
Deprecated as of 7.x-2.0.
Alter the default fields right before they are cached into the database.
@param array &$fields
By reference. The fields that have been declared by another feature.
Usually an alter hook is paired with a "normal" hook, but that is not always the case.
If you see that the hook is always invoked with
drupal_alter('field_default_fields', /* the other arguments */), the module uses only the alter hook. Checking if the module itself implements
hook_field_default_fields_alter() would also help to understand which hook is used from the module. Unfortunately, in this case, the module doesn't implement neither one or the other hook. This is easily explained from the fact the hook has been deprecated. (A module should be the first one not to implement its own deprecated hooks.)