Hi, 

So the only two boxes not explained are those in the grey area, right. ? 

The grey area is the knowledge area under consideration and the two boxes are processes within the knowledge area. The darker grey box of the two represents the current process under discussion. The non-greyed box represents process within the knowledge area which is not under discussion but its input or output is related to the grey box. 

Best way to understand is to look a couple of process in any one knowledge area. Go to develop project charter process to understand the legend.