Heads up class clarification

[JP1738]

I went to look at national record related rules when I stumbled upon the answer to this question. This is Section 2, Page 8 "Dialing under the Index"

"DIALING UNDER THE INDEX
Contestants in Super Stock and Stock have the option of
dialing under their class’ assigned index. It is the responsibility
of each contestant to place the selected time on the
windshield and tower-side window prior to each round of
competition. Dial-unders may be changed between rounds,
including a rerun situation. All contestants not choosing to dial
under will automatically be handicapped on the basis of their
assigned class index. In cases where two cars of the same
class are paired, the race is conducted on a heads-up basis,
regardless of any dialing-under considerations, and breakout
rulings do not apply"