For what it's worth, it is nearly impossible to understand from the NPS website that not 1 but 2 permits are required to descend the Subway via Das Boot.
Zion Park Wilderness Information indicates two ways to hike the Left Fork... one from the bottom and back... and the other way from the top down.
So it would be reasonable to think that an Upper Left Fork (Das Boot) permit would get you a downstream exit. What a rude awakening when the ranger informs you that you don't have permission to complete the Subway. Instead walk your arse back up the canyon.
The permitting of Das Boot and Subway should be made more clear in the website.
Thanks to those of you that have contributed to this thread. Now I know.

