I read part of this in the primer but would like to slightly clarify the question by giving concrete examples. How do they determine if a ticket is Nested or it is Back to Back. What constitutes an airport in a similar location.
Here are my examples. Ticket (A) and (B)

Scenario 1
(A)YVR-SEA-SFO
(B) SFO-SEA-YYJ
(B) YYJ-SEA-SFO
(A) SFO-YVR

YVR and YYJ are only 40 miles apart

Scenario 2
(A) YYJ-SFO
(B) SFO-SEA-YYJ
(B) YYJ-SEA-SFO
(A) SFO-YVR

(same as number 1, but no similar transit airports)

Scenario 3
(A) CLM – SEA-SFO
(B) SFO-SEA-YYJ
(B) YYJ-SEA-SFO
(A) SFO-SEA-CLM

Two end points YYJ and CLM are in different countries, but again only 20 or so miles apart


1) My understanding is that they are nested if the two turn around cities are far enough apart. How far is far enough apart?

2) How is a lay person supposed to know as I almost did this a couple of years ago before I heard about flyertalk in a legitimate run (had to escort grandma to SFO, get back to YYJ and then get back to SFO to take her back to YVR?
.

3) I was told back to back is fine if I used two different carriers (in this case AC & AS for example) What about if I used NW and AS??