Now let's talk about the most confusing part - the different accuracy parameters and their names.
CEP - Circular Error Probable. This is the size of the circle into which a certain percentage of solutions fall. If no number is specified, it's CEP50, meaning that half of the solutions should fall within the circle. If "accuracy" is mentioned without any specifics, it's also CEP50, as the smallest possible option. R95 is CEP95 (equal to CEP50 multiplied by 2.08), and R99.7 is CEP99.7 (equal to CEP50 multiplied by 2.90).
RMS - Root Mean Square. As Wikipedia states, it's the "square root of the arithmetic mean of the squared values." To be more precise, RMS is for one coordinate, and in a plane, it's called DRMS. It is equivalent to CEP50 multiplied by 1.2. The three-sigma rule works a bit differently here since it's for a plane. CEP50 is DRMS * 0.833, CEP95 is DRMS * 1.73, and CEP99.7 is DRMS * 2.41.
DOP - Dilution of Precision. This is a family of coefficients showing how the error in measuring pseudorange to a satellite affects the distance. It's an a priori value that depends only on the geometry of satellites in the sky. PDOP is used for a plane. They are used as follows: If a receiver's specification states that CEP50 is 3 meters at PDOP 1.0, then at PDOP 2.0, the CEP50 value will be 6 meters.
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