HP -67 Owner's Handbook Manual page 247

Using the I-Register for Indirect Control 243
We could also have used a subroutine for the digits for 100 (thatis,
2) in steps 002-003, 008-009, 019-020, and 038-039, but we
have used this morestraightforward program to illustrate the use of
the [@ instruction.
When you key in a four-digit seed numberin one of the three formats
§hown, an address(1, 2, or 3) is placed in the I-register. This address
18 used by the ([} instruction in step 027 to transfer program
execution to the proper routine so that the new random numberis
seen in the same form as the original seed.