Both constants have the same description:
ZBX_UNITS_ROUNDOFF_MIDDLE_LIMIT
Number of digits after comma, when value is less than roundoff threshold

ZBX_UNITS_ROUNDOFF_LOWER_LIMIT
Number of digits after comma, when value is less than roundoff threshold

Also seems all ZBX_UNITS_ROUNDOFF_*_LIMIT can be used also not only with described in the documentation options.