In addition to gross imports, UNSD recently introduced re-imports as trade flow. Re-imports is included in gross imports.
Note: in case your UN COMTRADE quick queries return trade values when the selected reporter is the same as the partner (China-China for example), what is returns by WITS is re-imports. If re-imports is not reported by a country, such query should return an empty table.
WITS Online Help
The World Bank, 2010