When your ESXi host experiences a problem while connecting to a storage device, the host treats the problem as permanent or temporary depending on certain factors.

Storage connectivity problems are caused by a variety of reasons. Although ESXi cannot always determine the reason for a storage device or its paths being unavailable, the host differentiates between a permanent device loss (PDL) state of the device and a transient all paths down (APD) state of storage.
Permanent Device Loss (PDL)
A condition that occurs when a storage device permanently fails or is administratively removed or excluded. It is not expected to become available. When the device becomes permanently unavailable, ESXi receives appropriate sense codes or a login rejection from storage arrays, and is able to recognize that the device is permanently lost.
All Paths Down (APD)
A condition that occurs when a storage device becomes inaccessible to the host and no paths to the device are available. ESXi treats this as a transient condition because typically the problems with the device are temporary and the device is expected to become available again.

Connectivity Problems and vSphere High Availability

When the device enters the PDL or APD state, vSphere High Availability (HA) can detect connectivity problems and provide automated recovery for affected virtual machines on the ESXi host. vSphere HA uses VM Component Protection (VMCP) to protect virtual machines running on the host in the vSphere HA cluster against accessibility failures. For more information about VMCP and how to configure responses for datastores and virtual machines when the APD or PDL condition occurs, see the vSphere Availability documentation.