The notation denotes the literature's exact subject. It consits of a 3- to 4-digit combination of letters referring to the library's classification scheme. The notation is therefore also refered to as the "classification mark".
All notations are defined in the the classification scheme (= Aufstellungssystematik). You can use the notation in the library catalogue for a subject-matter search as books related to the same subject are dedicated to the same notation.
The notation is also part of the shefmark. Since the books are placed in the shelves according to their shelfmarks and therefore to their notations, all books of the same subject are standing next to each other (provided they got the same location mark).