HP 35s Scientific Calculator - Manual (Page 228)

14-18 Programming Techniques
Loops with Counters (DSE, ISG)
When you want to execute a loop a specific number of times, use the  
(increment; skip if greater than) or   (decrement; skip if less than or equal
to) conditional function keys. Each time a loop function is executed in a program, it
automatically decrements or increments a counter value stored in a variable. It
compares the current counter value to a final counter value, then continues or exits
the loop depending on the result.
For a count–down loop, use   variable
For a count–up loop, use   variable
These functions accomplish the same thing as a FOR–NEXT loop in BASIC:
 variable = initial–value  final–value  increment
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 variable
A DSE instruction is like a FOR–NEXT loop with a negative increment.
After pressing a shifted key for ISG or DSE (   or   ), you will be
prompted for a variable that will contain the loop–control number (described
below).
The Loop–Control Number
The specified variable should contain a loop–control number ±ccccccc.fffii, where:
±ccccccc is the current counter value (1 to 12 digits). This value changes with
loop execution.
fff is the final counter value (must be three digits). This value does not change
as the loop runs.
An unspecified value for fff is assumed to be 000.