Talking about people playing as a “Number 9” vs. a “Number 8” or “6” or whatever.
Ah, OK. So basically, start in the back, left to right when looking from behind the GK. The wonkiness gets when you play a non-traditional formation.
9 is traditionally your center forward/striker. Your traditional 9 is big and strong and is trying to score anytime they get the ball near the box. Harry Kane and Erling Haaland are traditional 9s. This is definitely how Meinhard was being used at TU. You'll hear people refer to false 9. This is more like an additional MF, comes closer to the defense and helps pivot and distribute the ball. Firmino when he was at Liverpool is a good example. If you watch Ted Lasso, Jamie Tartt in season 2 was used as a false 9 in Nate's scheme.
Right now the US is playing a 4-3-3. The 3 MFs traditionally are the 6-8-10. Reyna is definitely the 10, but with Tyler Adam's absence, the US has actually been playing more of a two-6 system with Musah and McKennie acting as a double pivot connecting the defensive line to the attackers.
When Klinsmann was coach, the US played what would have traditionally been described as a 4-4-2 with the 4 MFs taking the shape of a diamond, dropping the 9 into that false 9 role. It worked on occasion but was really vulnerable against pressing teams as it focused too much on building through the center of the field contrasted to now where it's obvious to see where the US' primary talent ant strengths are on the outside and utilizing the full backs (outside defenders) pushing into the attack and connecting with your 7 (Pulisic) and 11 (Weah). And Berhalter's attack definitely utilized the 9 as a true 9. If you notice the play usually tries to get started with the 6, transition to offense through the MF in the center with McKennie or Reyna and getting the fullbacks on overlapping runs. The individual skill of the the US' current crop of 7, 9, 11, 8 and 10 give opponents fits and thus you've got Jedi and Dest finding a lot of space to them and becoming extra dangerous.
When my son was 11, Donovan Ricketts gave each kid on the team a 3x5 index card with their position number and a list of responsibilities/general positioning. It was a great way to introduce the boys to their roles but also who they should remain "in connection" with on the field.