Ceph » rgw

In RGWZoneGroup::dump method
encode_json("default_placement", default_placement, f);
this will call
void encode_json(const char *name, const rgw_placement_rule& r, Formatter *f) {
encode_json(name, r.to_str(), f);
}

std::string to_str() const {
    if (standard_storage_class()) {
      return name;
    }
      return to_str_explicit();
    }

std::string to_str_explicit() const {
      return name + "/" + storage_class;
    }
so, "default_placement" contains placement name and storage class.

But in RGWZoneGroup::decode_json method
JSONDecoder::decode_json("default_placement", default_placement.name, obj);
JSONDecoder::decode_json("default_storage_class", default_placement.storage_class, obj);
"default_placement" just decode to placement name, and there is no "default_storage_class".