[Louis Brunner]

12.5.1. However when the disc is in the air a player may not move in a manner solely to prevent an
opponent from taking an unoccupied path to make a play on the disc.

I think every elite runner is doing exactly what is explained, so just delete the rule

[Meret Trapp]

Boxing out is not illegal. The player needs to also make a play on the disc, either attempt to catch or to block. (That can happen after you moved aka boxed out your opponent.) “Solely” is the operative word there.

[John Kofi]

I guess if you move to occupy the path towards the disc, it makes it a legal box out.
It is illegal to only block a player without making a play on the disc. 😉

[Enrique Cipriano]

Apparently, I have misunderstood the rule for years. I teach my players that, unlike other sports, blocking is not allowed in ultimate. I cannot use my body to interfere with the path of a player making a play for the disc along a previously unoccupied path. Is Meret’s interpretation really correct?

[Michael Deters]

If you need reassurance on that – yes.

The annotations say the exact same thing Meret said:
(Look at 12.4) „The key word in this rule is ‘solely’. The intent of the player’s movement can be partly motivated to prevent an opponent from taking an unoccupied path to the disc, so long as it is part of a general effort to make a play on the disc.“

[Meret Trapp]

And as long as our movement does not result in a blocking foul. So it is legal to move to prevent an opponent form moving towards the disc as long as that opponent can avoid immediately running into you. Remember the sudden tree example.

But so long as you make a play on the disc and do not initiate contact via a blocking foul, you can box out as many players as you want.