The 9 PIN zones cover the Indian states and union territories as follows:
The first three digits of the PIN represent a specific geographical region called a sorting district that is headquartered at the main post office of the largest city and is known as the sorting office. A state may have one or more sorting districts depending on the volumes of mail handled.
The fourth digit represents the route on which a Delivery office is located in the sorting district . This is 0 for offices in the core area of the sorting district.
The last two digits represent the delivery office within the sorting district starting from 01 which would be the GPO or HO. The numbering of the delivery office is done chronologically with higher numbers assigned to newer delivery offices. If the volume of mails handled at a delivery office is too large, a new delivery office is created and the next available PIN is assigned . Thus two delivery offices situated next to each other will only have the first four digits in common.
Each PIN code is mapped to exactly one delivery post office which receives all the mail to be delivered to one or more lower offices within its jurisdiction, all of which share the same code. The delivery office can either be a General Post Office (GPO), a Head Office (HO) or Sub Office (SO) which are usually located in urban areas. The post from the delivery office is sorted and routed to other delivery offices for a different PIN or to one of the relevant sub offices or branch offices for the same PIN. Branch offices (BO) are located in rural areas and have limited postal services