As far as I know, the plus-or-minus sign is used to indicate the statistical margin of error of a measurement. But what does it mean when there are two plus-or-minus signs in the same measurement?
Some examples from Wikipedia:
- The mass of M87* is given as
(6.5 ± 0.2stat ± 0.7sys) × 109 M☉
(as measured by the Event Horizon Telescope). - The measured distance to Sagittarius A* is given as
7860 ± 140 ± 40 pc
.