What is UID?
The UID is just the unique read only identifier that is on the tag. This number is unique no matter where you buy the chip. This is possible because the number is composed of a unique manufacturer number and a unique chip number. Also, each manufacturer is required to only program tags with unique numbers. No number can be used twice.
Here is a breakdown on how each UID is coded.
Usually the UID is not changeable. Only the UID of special card named MIFARE can be changed by end user.