That looks like an asymmetric error bar, meaning the best estimate is 6.2 but could be 2.2 above or 1.3 below.

Essentially it's a common physical scientist way of expressing a point estimate with a confidence interval. (In the life and social sciences they tend to just provide the point estimate and confidence interval explicitly.)

Seems like an uneven error interval? Meaning if it’s not our estimated valid of 6.2, we have less confidence in the upper bound than the lower bound, or our estimates are distributed nonuniformly

Planet Nine (supposedly) has a mass somewhere between 4.9-8.4x that of our own planet.