A call number can be a random number, but in modern libraries the call number is a classification number.
You may already be familiar with the Dewey Classification System, it's used in Hong Kong public libraries and in most school libraries, and in some other universities in Hong Kong. An outline of Dewey Classification categories is available from OCLC.
LC (Library of Congress) Classification System: Used by HKUST Library!
It was created by the Library or Congress in the USA. Most university and research libraries in North America use it. Here is an outline of the LC Classification from the LC's own home page.
Classification numbers are built in Dewey, they are assigned in LC.
Both systems try to bring together similar items.The similarity is based on the following criteria:
Fully detailed guide here: https://libguides.ust.hk/classification
Basic Outline of LC Classification is listed below. The links will bring you to the detailed outline for each letter.
A - General works
B - Philosophy, Psychology, Religion
C - Auxiliary Sciences of History (e.g., numismatics or genealogy)
G - Geography, Anthropology, Recreation
H - Social Sciences (including Economics)
J - Political Science
K - Law
L - Education
M - Music
N - Art & Architecture
P - Language & Literature
Q - Science & Math
R - Medicine
S - Agriculture
T - Technology & Engineering
U - Military Science
V - Naval Science
Z - Library Science, Publishing, Bibliography, Information Sources