In e-discovery, electronically stored information (ESI) is divided into five categories, which are grouped into two tiers based on the effort and cost needed to access ESI. Keep these categories in mind when requesting ESI or responding to a request:

Category What It Is Accessibility
Active, online data ESI created, received, or processed; or that’s quickly and frequently accessed. Examples: hard drives and active network servers. First tier: reasonably accessible
Near-line data (short for near online) ESI stored on removable media or accessed via automated or robotic storage systems. Access speeds range from a few milliseconds up to 2 minutes. Examples: optical disk and magnetic tape. First tier: reasonably accessible
Offline storage and archives ESI sent to storage. Unlike the first two categories, offline ESI is accessed manually. Examples: magnetic tape or optical disks; referred to as JBOD (just a bunch of disks). First tier: reasonably accessible
Backup tapes, commonly using data compression ESI stored for backup or disaster recovery and not organized for retrieval of specific files or messages. Retrieving ESI requires restoring the entire tape and might require reversing the compression used to fit more bytes of data. The discovery of ESI from backup tapes requires proof that their need and relevance outweigh their retrieval and processing costs. Example: backup tapes. Second tier: not reasonably accessible
Erased, fragmented, or corrupted data Erased, overwritten, fragmented (broken up and stored in separate areas), or corrupted files (damaged by computer viruses, or a hardware/software malfunction) are the least accessible. This ESI might be accessed only after significant processing or might be impossible to access at all. Second tier: not reasonably accessible