IOCTL的I/O控制疑问

_IOR(base, command, size)

/*
* _IOR means that we're creating an ioctl command
* number for passing information from a user process
* to the kernel module.
*
* The first arguments, MAJOR_NUM, is the major device
* number we're using.
*
* The second argument is the number of the command
* (there could be several with different meanings).
*
* The third argument is the type we want to get from
* the process to the kernel.
*/
没看懂第二个参数是什么意思啊？有大牛了解吗？

以_IOR('V',  0, struct v4l2_capability)为例，第一个参数是幻数，第二个就是命令，第三个是传替参数的大小。第二个参数根据不同的命令可以定义为任意数字，这个就看个人喜好了。比如：
#define cmdA   _IOR('V',0,struct v4l2_capability)
#define cmdB   _IOR('V',1,struct v4l2_capability)
#define cmdC   _IOR('V',2,struct v4l2_capability)
#define cmdD   _IOR('V',3,struct v4l2_capability)