What does A42 as a reason for being denied entry into Canada from the US by a US national (born and raised in US) mean?

I can think of reasons my nephew was denied, such as driver's license from NY but entering from Montana where he just moved to, no current job (lots of money from computer consulting), and it was the day before 9/11, but I just want to know what A42 stands for and can't find it. Poor guy was on his way to his cousin's wedding.

Does he have any convictions, even misdemeanors?

No.
But I don't feel I need to go on about him; he's squeaky clean. I just want to know what an A42 is.

Did he have his passport with him at the time. Since you need a passport, not a drivers license

He has a passport and is very smart; I'm sure he had it. And he tells me his car and license are now in MT.
Here's what I think:
Tiny border crossing at MT.
Day before 9/11.
Self employed computer consultant.
Long black hair and looks native American or even possibly middle eastern, or any nationality.
Last trip abroad: I think Egypt (to see the pyramids).

My sister and BIL in WA state went to the same wedding and got grief at the big crossing too. My sister said 'Good morning' far too cheerfully instead of waiting to be spoken to. The guard just stared at her for many minutes (it seemed) and then shot out a string of questions she couldn't answer, and when she finally got all flustered he let them through.

ANYWAY I'm still just curious about what an A42 is. My nephew wants to drop the subject. He drove and drove and had to drive home and didn't get to see any family.

Enforcement Manual ENF 2, Section 11:

http://www.cic.gc.ca/manuals-guides/english/enf/enf02e.pdf

"A42 stipulates:

42. A foreign national, other than a protected person, is inadmissible on grounds of an inadmissible family member if

(a) their accompanying family member or, in prescribed circumstances, their non-accompanying family member is inadmissible;"

I don't understand. He was alone, and no family has ever been inadmissible.