The Pad to Pad (PP) distance is specified in mm and is one of the parameters that determines the pattern class of the board.
A typical PP value is 0.150mm or 6mil. This is the value for our pattern class 6 our standard.
We define OPP as Outer layer Pad to Pad distance and IPP as Inner layer Pad to Pad distance
Insulation is defined as
The insulation distance may not be reduced to less than 75% of the nominal distance (including unwanted random particles).
R1 > 0.75 A1
R2 + R3 > 0.75 A2