Global Dimension 1 is Shortcut Dimension 1, same with 2.
The shortcut dimension appears as standard with an appropriate label in all journals and transaction screens, dimension 3 does not, you need to add it through the line if you want to manually add it to the transaction.
The line and header differentiation will be different where the characteristics are different. If GD1 is customer related and GD2 item related then teh Header would show GD1 and GD2 would be blank, then every line will have the same GD1 but potentially a different GD2 for each line. Enabling you to slice your data.
It depends how you set your dimensions up, department and project would potentially always show the same, but then if you added a third dimension to a single line you would see a difference.