At UTS the subject codes are five or six digits long, and I've figured them out a little. First number is Faculty or School (eg 6xxxx for Science, 3xxxx for Maths and IT) with some logical inconsistencies based on historical relations. Second digit indicates the particular department (68xxx = Physics, 65xxx = Chemistry) and the remaining three digits indicate (in variable combinations) the specific field and level of the subject.
At UWA the subject codes seem to function on the basis of six digits seperated into threes by a point or a slash. The first three indicate the faculty and school (eg I'm currently guessing 909 to be Medicine) and the last three designate the field and level of the particular subject, as well as the semester in which it takes place. Exempi gratia: Foundations of Clinical Practice 1 is 909.111 (semester 1), Foundations of Clinical Practice 2 is 909.112 (semester 2).
I personally prefer the numerical system, it seems neater and more logical than alpha codes (many more numbers available) but ONLY when logical consistency is maintained. If you allow historical links to interfere with the numerical logic of the codes then the system is complicated by ambiguities (such as the confusion between Maths and IT subjects at UTS, caused by the era in which the Faculty of IT was the School of Computer Science, in the Department of Mathematics).