When you create a Timeline Matrix, you choose between two starting points: Row Query or Cell Query. The matrix page tabs and the tasks in the general tab change as you swap between the starting points.
A single Query is used to populate the rows with Elements. The columns defaults to the months of the current year.
This starting point is more complex. It starts with a Query to identify the cells, and then uses additional Row and Column Queries to relate the cells to respective row and column Elements.
The matrix tab has a 'Change' button to move between the different starting points. This button is located on the right hand side of the screen beneath the diagram of the starting point.
Changing the starting point is demonstrated in the short video clip below: