ESP-WROOM-02D and ESP-WROOM-02U are ESP8266EX-based modules developed by Espressif.
Compared to ESP-WROOM-02, the RF performance of ESP-WROOM-02D and ESP-WROOM-02U are optimized. Besides, ESP-WROOM-02U integrates a U.FL connector. Please see Chapter 8 for details of U.FL connector.