Thanks for you reply! I was waiting and hoping a few more people would comment but let's discuss your reply.
1. Why does this not apply?
2. Makes sense
3. And if contact is involved?
1. I would suggest that it does apply in some situations but not in others. Generally speaking, enforcement of ‘principle of verticality’ rules tend to hinge on whether or not the disc was in your ‘airspace’ and whether or not the contact was incidental. Making calls on these rules, perhaps more than others, is complicated and rather subjective.
When two opponents compete for a disc in the air, a certain amount of contact is usually unavoidable. Shoulders, arms, legs, hips, and hands often bump when two people get up for a disc, but it doesn’t affect the outcome of the play. That is incidental contact, and a certain amount of that must be tolerated, because most of the time that contact between two opponents is equally attributable to each.
If you are certain that the disc was exclusively in your ‘airspace’ and that your ability to make the catch was impeded by contact from an opponent, it is a foul. Your decision here depends on the nature of the contact – was it incidental or not.
If the disc was not exclusively in your ‘airspace’, then the principle of verticality no longer applies, and there is typically going to be contact more frequently because both teams have the right to make a play on the disc. When two opponents chase down a disc, paths will intersect! Deciding how much contact is tolerable between opponents is a judgment call, typically coming from experience.
But always call a foul if:
a) The non-incidental contact was initiated by your opponent.
b) The play was reckless or dangerous – don’t let yourself be put at risk by playing against goons.
2. In my opinion:
a) if a player is in a stationary position, he/she has the right to the airspace.
b) if the players are both moving toward an unoccupied space, the player who arrives there first has the right to the airspace (unless that play is reckless or dangerous).
c) if both players arrive at the same time, then both of them have the right to the airspace (unless that play is reckless or dangerous) and it looks like there is going to be a train wreck (but the contact is no more attributable to one than the other so be wary about calling fouls here).
Think of it like this: did your opponent contact you (foul on him), did you contact your opponent (foul on you), or did you and your opponent contact each other (foul on nobody)?
3. If there is no contact, there is no foul - you've been "skied". If only incidental contact results (contact that did not affect the play such as gentle bumping after the catch) then there is no foul. If non-incidental contact results (play was affected, i.e., you were physically prevented from making a play) call a foul on your opponent.