Each header part (row) has three columns and their widths proportions may be set in two variants.
First is called “Flexible middle column” and is set by default for all rows. The main feature of this variant is that left and right columns will take their width from the elements’ width added inside them. And the middle one will be flexible between them and take all the free space. This layout is preferable if you place your logo image to the left column, additional icons to the right column and your main navigation element takes the full width of your middle column.
Rows design examples: