The starting index value

(Display Figure 7.)

Note the little triangle following the 0 on the left side of Figure 7.

Clicking that triangle exposes a drop-down list that lets you specify a numeric value other than 0 as the starting value for the counter variable. 

In addition, you can specify an expression, including the name of a variable instead of a literal numeric value.

Note however that the expression must evaluate to a value of type Number.