Here is what the "EXTEND" menu is:
EXTEND: This sets the frequency of when the game awards extra lives.
The settings are:
TYPE 1: Extra ship every 2,000,000 points.
TYPE 2: First extra at 3,000,000 points, and then every 5,000,000
points thereafter.
TYPE 3: First extra at 6,000,000 points, and then the second extra at
15,000,000 points.
NONE: No extend.
I hope this helps!