1, network type: refers to downloading and reading e-books directly through the Internet without using any equipment.

2. CD-ROM type: refers to storing e-books on CD-ROM, and you need to use CD-ROM drive to read them.

3. Hard disk type: refers to storing e-books on a hard disk, which requires a hard disk drive to read.

4. Mobile storage type: refers to that e-books are stored on mobile storage devices, such as USB flash drives and mobile hard disks, and can only be read by using corresponding devices.