There are 68 module positions. If no module is published to a position, it will not appear, collapsing the entire area.
Non-Standard Elements
There are non-standard elements that are injected into the template grid structure when enabled via the template parameter, such as the logo into header-a. These features stack vertically with any modules published to that position.
You can change the positions of these elements at Extensions → Template Manager → rt_nebulae_j15
Scrolling Modules
Scrolling Positions is a feature of Gantry, which, when activated, modifies the behaviour of stacked modules. Instead of appearing vertically, the additional modules assigned to a given position become accessible via a javascript transition button, shown on the frontpage as: "More".
The Content Top and Content Bottom position rows have this feature.
Layout Control
Control all modules positions with Gantry, in terms of widths within the module row, and left/right/middle placement for the mainbody/sidebar area.
Gantry Framework : Layouts Documentation
Preview
View all module positions live by appending ?tp=1 or &tp=1 to the end of your URL such as http://yoursite.com/index.php?tp=1.
The below diagram is of the Module Variations page.