ESP-12(E/F/S) is the most common Wi-Fi module format and is pin compatible with other non-Espressif modules such as WB3L, WB3S, WR3, WR3E, WR3L, WR3N, CB3S, CB3L, C-8138 and more…
ESP-12E/F modules need the GPIO15 pulled low and CH_PD or EN pin pulled high to boot.
ESP-12S modules have those pins already pulled internally.
Serial Flashing ESP-12S
Serial Flashing ESP-12E/F