The left side panel holds an optional fan hole (120/140mm) whereas the right panel is fully closed.
The left side is the so called ModuVent, which means that the company used removable seals to close optional holes, until you need to use them. This is how Fractal Design managed to make the case as silent as possible.
The following photo shows the sound isolation material in the Black Pearl. The side panels are tough and are easily taken off. They’re held in place by two thumb screws.