The key-of-reference specifier can optionally accompany the key- field-value specifier. The key-of-reference specifier indicates the key-field index that is searched to find the designated key-field value. It takes the following form:
A value of zero indicates the primary key, a value of 1 indicates the first alternate key, a value of 2 indicates the second alternate key, and so forth.
If no kn is indicated, the default number is the last specification given in a keyed I/O statement for that I/O unit.
For More Information:
For details on the key-field-value specifier, see Section 10.2.1.5.