In the ESXi context, the term target identifies a single storage unit that your host can access. The terms storage device and LUN describe a logical volume that represents storage space on a target. Typically, the terms device and LUN, in the ESXi context, mean a SCSI volume presented to your host from a storage target and available for formatting.
Different iSCSI storage vendors present storage to servers in different ways. Some vendors present multiple LUNs on a single target, while others present multiple targets with one LUN each. While the way the storage is used by ESXi is similar, the way the information is presented through administrative tools is different.
Three LUNs are available in each of these configurations. In the first case, the host detects one target but that target has three LUNs that can be used. Each of the LUNs represents individual storage volume. In the second case, the host detects three different targets, each having one LUN.
Host-based iSCSI initiators establish connections to each target. Storage systems with a single target containing multiple LUNs have traffic to all the LUNs on a single connection. With a system that has three targets with one LUN each, a host uses separate connections to the three LUNs. This information is useful when you are trying to aggregate storage traffic on multiple connections from the host with multiple iSCSI HBAs, where traffic for one target can be set to a particular HBA, while traffic for another target can use a different HBA.