[Why can't I set my new document to 600 dpi?]

Thanks for the help. I didn't realize you could type in that box.

[Why can't I set my new document to 600 dpi?]

This is for printing a comic book.

According to: https://mckelvie.tumblr.com/post/167235929653/comic-book-page-technical-specifications

My page needs to be set up as follows:

6.875 by 10.438 inches 

When I set up my document in Designer or Photo the max dpi is 400. The article says it needs to be 600 dpi in some cases. Also on Comixology's page they allow documents up to 600 dpi as well. I'm told that the higher the dpi the better.

So why can't I choose 600 dpi on the slider? It maxes out at 400.