For pipelines, this value is derived from Barlow's Formula, which takes into account wall thickness, diameter, allowable stress (which is a function of the material used), and a safety factor.
The MAOP is less than the MAWP (maximum allowable working pressure).
MAWP is defined as the maximum pressure based on the design codes that the weakest component of a pressure vessel can handle.
Due to the availability of standard wall thickness materials, many components will have a MAWP higher than the required design pressure.
This is +10% for PD 5500, and ASME Section VIII div 1 & 2 (with an additional +10% allowance in ASME Section VIII for a fire relief case).