The next value is somewhat tricky. This takes us into an entirely different topic of scaling and dimensioning, which I'll postpone for a later time. If your Units settings is set to Architectural or Fractional, you have control over the size of the fraction relative to the dimension text height. This is a scale factor, hence .5 would create fractions one half the size of the whole numbers. The value is saved in the dimtfac system variable.
Some applications require a frame to be drawn around the dimension text.
So, selecting the check box will generate a box around the text and set the dimgap variable to a negative value. This part starts to get a tad sticky. You can control the vertical and horizontal justification of text along the dimension line. This setting will affect just about all dimensions. Above will place the dimension text above the dimension line.