The graphic above represents the size of your matrix as viewed from the back side. The outline of each panel is in grey. The green item on the left is a computer mount that will secure your computer to the back of the matrix. The optimal position of the computer is depicted correctly.
The red crosses at the joint of each panel is representing an X mount. These X mounts allow the panels to be mounted side to side, top to bottom. The light blue items on the outside joints represent Ts. The Ts allow for panels to be secured on outside edges and have extra material to allow for mounting to an enclosure. The X and T system does not work with indoor P5 panels. They are only compatible with indoor P10 panels and outdoor panels. Sometimes this system costs less, and sometimes it costs more, it depends on your panel size.
The red lines down the center of each joint between the panels represents a center mount strip. This allows the panels to be screwed together side by side and have extra material at the top to allow for mounting to an enclosure. The light blue strips on the left and right of the panel represent side mounting strips. They come in a pair. They allow you to mount the sides of a panel to an enclosure.
The red dots in the four corners of each panel represents a set of magnets. The magnets will hold the panel to a steel structure allowing the panel to be serviced easily without unscrewing the panel from a mounting plate.