Hi Nidhi, Autopk="true" is used to declare a unique key. Read below -
autopk (boolean): if this option is activated (autopk="true"), a unique
key will be automatically defined. This option may only be used on the
main element of the schema. Warning, Adobe Campaign only guarantees that
the key generated is unique. It is not guaranteed that the key values
are consecutive and incremental. ThanksDeepak