function of major/minor device number

Hi, all I have a question about major/minor device number. I know that one can use int register_chrdev_region(dev_t from, unsigned count, const char *name); to register a driver with a unique pair of major/minor device number. But what's the difference between two drivers: in one case they have same major device number but different minor device number; on another case they have different major device number but same minor device number. When I searched the Internet, it said that major device number is to identify the driver while the minor device number is to identify specific hardware. But I'm still confused because it seems legal to share the major device number with different drivers. Please help me！Thank you!