Example: 312.5634 H28
There are three separate areas to look at:
312 is the subject classfication number
5634 comes after the decimal point and each separate number is taken into consideration individually. For example, 312.5634 would come AFTER 312.23799.
The third part is the H28. It is read the same way as the second part, except that it is preceded by a letter, which is read alphabetically as well.
Here is a correct arrangement of some call numbers: