What is a "line index"?

[Matthew Renner]

When referencing terms, sometimes it is said "Line index must be from 0 to 7. What is "line index" and what is each number mean (0 to 7)?

[Al W]

If there are 8 lines to choose from, the line index of the first line will be 0 and the last will 7.

[meikel]

in each indicatorwindow you can draw a maximum of 8 lines.

each can referenced by a number, the "index"

0-7 = 8 lines