Не помню где взял, но откуда-то для себя скопипастил: GD32F303xx microcontrollers where the flash memory density ranges between 128 and 512 Kbytes
are called High-density devices (GD32F30X_HD).
GD32F303xx microcontrollers where the flash memory density is over 512 Kbytes
are called Extra-density devices (GD32F30X_XD).
GD32F305xx and GD32F307xx microcontrollers are called connectivity line devices (GD32F30X_CL).