The letter after the decimal point is the first letter of the name of the journal. There are many exceptions to the above rules. Here are some of them. If the subject, like topology, is further subdivided, the first part of the number may contain itself a decimal point, for instance QA611.5.E7 . Do not expect too much logic in assigning the subject to a journal.
Many journals have more than one name.
It is impossible to guess what name will be used for classification. Sometimes classification according to the name is difficult to explain.