在STM32的初始化文件中看到一段说明：

 - For Low, Medium and High density Value line devices an external 8MHz
          crystal is used to drive the System clock.
        - For Low, Medium and High density devices an external 8MHz crystal is
          used to drive the System clock.
        - For Connectivity line devices an external 25MHz crystal is used to drive
          the System clock.
     If you are using different crystal you have to adapt those functions accordingly.


请问一下：High density Value line device 和Connectivity line device 这两个有什么区别？是什么意思啊？