There are 68 collapsible module positions. If no module is published to a position, it will not appear, collapsing the entire area.
There are non-standard elements that are injected into the template grid structure when enabled via the template parameter.
For example: The logo element is injected into the header-a position. It is inserted in a stacked fashion, meaning, if there are modules published to the header-a position, they will appear in the same column as the logo, just one place below.
You can change the positions of these elements at Extensions → Template Manager → rt_juxta_j15
The positions themselves are highly configurable, with the ability to change the width allocation depending on how many positions are published to a particular module row, such as feature-a/f. This is also applicable to the mainbody which, provides for an easy mechanism to switch the positions and widths of the side columns. For more information, check the documentation linked below.
[readon2 url="http://www.gantry-framework.org/documentation/joomla/configure/layouts"]Gantry Framework : Layouts Documentation[/readon2]
The below diagram is of the Module Variations page.