A grading scale is simply a mapping of each course's letter grade into a numeric grade. Since a GPA is a number, each letter grade your student receives needs to be mapped to a number before the GPA can be properly calculated.

There are various grading scales in use by different schools. The most common is the 4.00 scale. This means letter grade A translates into a 4.00 numeric grade:

A => 4.00
B => 3.00
C => 2.00
D => 1.00
F => 0.00

Another common scale is 4.33 which is useful for mapping letter grades with pluses and minuses (some schools don't use pluses and minuses, but for those who do this would be the correct scale to choose):

A+ ... 4.33
A ..... 4.00
A - ... 3.67
B+ ... 3.33
B ..... 3.00
B - ... 2.67
etc.

The grading scale in use is displayed next to the GPA on the printed transcript:

If you need to use plus/minus grades (e.g. A+, B-) on your transcript, you should select a plus/minus grading scale in the "Settings" tab. If you don't need to use plus/minus grades, the default 4.00 scale will work for you.

Note that the grading scale has nothing to do with GPA weighting. A weighted GPA can be used with a 4.00 or a 4.33 grading scale. Just because your transcript uses a 4.33 grading scale does not necessarily mean a weighted GPA is being calculated.