HBOUNDS_TEXT will only be used if
Some fonts have characters that will extend below the baseline. For example, p, q, or y all have tails that extend below the baseline.
The text in HBOUNDS_TEXT will be used to attempt to figure out the lowest and highest point of all the characters. This will create a uniform height across all generated images for a particular font size.
You should not have to change this value unless you are working in a language that does not use the a-z alphabet.