RNP is a required performance that your aircraft has to be able to match, it is lets say a tube, with a diameter, in our case the diameter is equal to 0.3, so if the sensors plus the other errors of your aircraft display 0.3 or less, then the aircraft, that we assimilate to a ball, can enter the tube and go through it.
diameter or radius?
Im not sure the tube theory is a particularly good analogy for approaches. Being 0.3 vertically from design track centreline is not acceptable. 0.3 laterally and 75' vertically is the limit for my company.
Confusion about the 0.6 bit could come from the idea that for procedure design, obstacles are considered out to 2 X RNP value from the design centreline (.3 X 2, in this case).