What do superscripts and subscripts in statistical notation mean?

[plasmon]

I have studied a measurement 2(+0.9 error(written in superscript) and -0.1 error(written in subscript)) in a book that gives statistical value of difefrent parameters . What does this measurement implies. As a normal statistical error is written as (2 +-0.9).
For e.g
_²-0.1+0.9

 

[HallsofIvy]

My best guess is that it is referring to a "non-symmetric" interval: that is
$$2_{-0.1}^{+0.9}$$ refers to a number that may be as low as 2-0.1= 1.9 or as high as 2+ 0.9= 2.9.