http://time.com/4955704/nfl-league-rulebook-a62-63-national-anthem-rule/
Rush Limbaugh referenced he had a copy of the specific NFL national anthem protocol on his radio show this past Monday:
The problem is, there is a rule in the NFL that players must stand, must be on the sideline. I’ve got the rule right here. You have to be there. But rules are shmules, right? There aren’t too many rules that are enforced these days. The comment that Tomlin made that just made me curious was “protect.” “I want to protect the players from having to make a choice.” Well, making a choice is what they’ve been doing. What he wanted to do was protect the Steelers organization from being part of this. That’s admirable.
https://www.rushlimbaugh.com/daily/2017/09/25/interesting-takes-on-trump-and-the-nfl/
Like most sports leagues, the Commissioner can make things up on the fly anyway.
The ‘Game Operations Manual’ is not made public and there is no indication that it is mandatory or that the supposed quotes from it are remotely current.