information on this is very sparse in game, i only figured it out by happenstance when i saw some build on the online tool using skills from multiple specs.
But yeah, its visualized in the tree as a chain going through the middle. You cannot get passives or skills past this point.
This kinda bothers me a bit. Every character you level has a large pop up that you have to manually close which explicitly says this information. On top of that, it is also clearly detailed in the game guide. And I'm pretty sure that each node also says what they require.
Edit: so I know how much hand holdy force tutorials suck but what's the alternative here? Writing it down in multiple places and forcing the player to accept it clearly doesn't work. Should I be recording my voice to come through in the tutorials?