In HTML5, I know that can be used either inside or outside the page\'s masthead element. For websites having both secon
@IanDevlin is correct. MDN's rules say the following:
"The HTML Header Element "" defines a page header — typically containing the logo and name of the site and possibly a horizontal menu..."
The word "possibly" there is key. It goes on to say that the header doesn't necessarily need to be a site header. For instance you could include a "header" on a pop-up modal or on other modular parts of the document where there is a header and it would be helpful for a user on a screen reader to know about it.
It terms of the implicit use of NAV you can use it anywhere there is grouped site navigation, although it's usually omitted from the "footer" section for mini-navs / important site links.
Really it comes down to personal / team choice. Decide what you and your team feel is more semantic and more important and the try to be consistent. For me, if the nav is inline with the logo and the main site's "h1" then it makes sense to put it in the "header" but if you have a different design choice then decide on a case by case basis.
Most importantly check out the docs and be sure if you choose to omit or include you understand why you are making that particular decision.