Does a name go before or after the noun it modifies?
The sentence
The user “john_smith” has been registered; go to the “User Profile” tab to view the user’s details.
reads more naturally to me than
The “john_smith” user has been registered; go to the tab “User Profile” to view the user’s details.
but I’m not sure why. In particular, it seems wrong for john_smith to go before the word user, whereas User Profile can go either before or after the word tab (although it seems to flow a little better when it’s before).
What rules of grammar apply here?
UPDATE 5/19/2018: I’ve changed “John Smith” to “john_smith” to help clarify that it is supposed to be a unique “username” assigned to a new user.
I disagree with most of what Luke Baumgarten says.
In general you can use any noun phrase to modify another noun in English (I don't think it is helpful to say it is 'treated as an adjective'): the dog basket, a big city attitude (where big city, a noun modified with an adjective, is the modifying noun phrase).
Personal names are less often used in this way, though they can be; but when they are it is to characterise the head noun as a kind of object, not an individual associated with that name. So a Ben Sherman shirt is a kind of shirt produced by the Ben Sherman company, (or if you know a Ben Sherman who is known for wearing a particular kind of shirt, then people in your circle might refer to a shirt of that kind as a Ben Sherman shirt). But you would not say a Ben Sherman shirt to mean any shirt which happened to be Ben's (though you might if it was one of his characteristic shirts - because it is a Ben-kind-of shirt, not a belonging-to-Ben shirt). For this reason you would be unlikely to hear the John Smith user (though not impossible). To refer to the user who is (or the user account which is used by) the individual John Smith, it is much more usual to put the name in apposition, the user John Smith.
(Place names on the other hand are used in this way: London policemen are policemen from London, not (generally) London kind-of policemen).
The User Profile tab is a slightly different case because User profile does not represent an individual, so the meaning of a User-Profile-kind-of tab is the only one available. (The fact that there might be only one such tab in a particular application is not relevant: User Profile does not identify anything individually in the sense that John Smith does.) The User Profile tab means the tab which is of a User Profile kind; the tab "User Profile" means _the tab which is named "User Profile": there may not be much practical difference.