Hi Mohammad,
A PDL is used for sending a message to groups of users in bulk.
Public Distribution Lists Overview
Public distribution lists are used to send voice messages to multiple subscribers. The subscribers that are assigned to a public distribution list typically are subscribers who need the same information on a regular basis, such as employees in a department or members of a team. The class of service that is associated with each subscriber account dictates whether subscribers can send messages to public distribution lists in Cisco Unity.
Cisco Unity automatically creates a public distribution list called All Subscribers. You may want to associate the All Subscribers list with every subscriber template.
http://www.cisco.com/en/US/docs/voice_ip_comm/unity/5x/administration/guide/ex/5xcusag240e.html#wp1047746
Hope this helps!
Rob
