Yes, we are talking about a sideline. The sideline is introduced by the move 9..h6; the main line is 9..Re8. After 9..h6 10.Bh4 Re8, the position is different. For one thing, with the Bishop on h4, it now has access to f2, where it is well placed to support central expansion.
IMHO, White's best move now is 11.f3. This can be debated, I know. Many players will look at the pawn on ..h6 and think that the idea of 0-0-0, h3 and g4-g5 now looks good. But I think there is a problem with this plan. Principally, if White plays 11.0-0-0, Black has 11..Ne4! which gives him good counterplay.
After 11.f3, I have 121 games in my database. 11..c5 is most popular, followed by 11..Nf8. The position I want to look at, though, arises after 11..Qc7, which has very rarely been played. So, we are actually in a sideline of a sideline!!