HP 35s Scientific Calculator - Manual (Page 233)

Programming Techniques 14-23
You can not solve or integrate for unnamed variables or statistic registers.
Program Control with (I)/(J)
Since the contents of I can change each time a program runs — or even in different
parts of the same program — a program instruction such as STO (I) or (J) can store
value to a different variable at different times. For example, STO(-1) indicates storing
the value in Variable A. This maintains flexibility by leaving open (until the program
runs) exactly which variable or program label will be needed.
Indirect addressing is very useful for counting and controlling loops. The variable I
or J serves as an index, holding the address of the variable that contains the loop–
control number for the functions DSE and ISG.
Equations with (I)/(J)
You can use (I) or (J) in an equation to specify a variable indirectly. Notice that
 or  means the variable specified by the number in variable I or J (an
indirect reference), but that I or J and or  (where the user parenthesis are
used instead of the (I) or (J) key) means variable I or J.
Unnamed indirect variables
Placing a positive number into variable I or J allows you to access up to 801 indirect
variables. The following example indicates how to use them.
STO(I)/(J)
RCL(I)/(J)
STO +, –,
Ч ,ч, (I)/(J)
RCL +, –, Ч ,ч, (I)/(J)
X<>(I)/(J)
FN=(I)/(J)
INPUT(I)/(J)
VIEW(I)/(J)
DSE(I)/(J)
ISG(I)/(J)
SOLVE(I)/(J)
∫ FN d(I)/(J)