Underlined vs non-underlined date fragments in match dialog

In the month, date and year drop downs when editing the token for Custom Date match some of the entries are underlined? I have not been able to find documentation on what this means.

My guess is that maybe it is to differentiate for days 1-9 being represented as 01-09? And similar for months 1-0 and years before 999?
I am having trouble getting date matching working but want to make sure I understand those options before posting a more in-depth query.
thanks,
chuck

My guess is that maybe it is to differentiate for days 1-9 being represented as 01-09? And similar for months 1-0 and years before 999?
I am having trouble getting date matching working but want to make sure I understand those options before posting a more in-depth query.
thanks,
chuck