Air gap on isolation space
A PCB will conduct across its surface due to minute contaminants. Placing an air gap between circuits that ought to be galvanically isolated extends the direct creepage distance i.e. any conduction will have to find a way around the air gap. This effectively makes the "surface" exclusion zone wider.

Note that the two air gaps are where components are closest facing each other across the 8mm barrier.





Here is a useful calculator that allows creepage and clearance to be calculated based on IEC60950