Asalam o alaikum,
I have a query regarding the sources of International Law. I read it somewhere that the sources of international law are traditionally characterized as formal and material sources.
Formal sources ( what the law is e.g treaties, custom and general principle) and material sources ( where it can be found e.g judicial decisions).

Now the problem is whether material sources of IL will constitute of all of these terminologies or just the clause d of Article 38 of Statute of IL?

p.s can anyone help me with the difference in custom and usage?

Regards

Formal sources:
They create IL.
Custom, Treaties, General Principles.

Material sources/subsidiary means:
These are law identifying sources. They DO NOT create IL, rather they are used for interpretation.
Judicial Decisions
Academic writings.

Now, material sources DO NOT include customs, treaties and GPs. If you include these in material sources, you will lose the Formal sources category (so you can't). However, do remember that judicial decisions n academic writings usually use the formal sources to reach a conclusion. These conclusions are based on interpretations and you CAN NOT find them as such in the Formal sources.

Coming to your last query:
Usage: it is the early form of Custom. It refers to the expectation of State A that State B will behave in a certain way in a particular situation. And State B repeats that behavior several times. It, however, has no legal validity.

Custom: When a Usage gets old, it becomes a custom. Put it STARKE's way: 'custom begins where usage ends.' But this is not an overnight transformation and needs:
1) long period of time (in which State(s) repeat that behavior pattern)
2) Uniformity and consistency (of the Usage in question)
3) generality of practice (many States behave that way)
4) opinio juris: States should believe that they are under legal obligation to follow the usage.

Finally, it's not mandatory for a usage to become custom. Furthermore, not every custom is necessarily preceded by a usage.

Starke has included all the formal sources in material sources (see Starke chapter 2). What I can conclude is that material sources DO NOT mean subsidiary sources.

"A distinction has sometimes been made between formal and material sources. The former, it is claimed, confer upon the rules an obligatory character, while the latter comprise the actual content of the rules.
Thus the formal sources appear to embody the constitutional mechanism
for identifying law while the material sources incorporate the essence or
subject-matter of the regulations. This division has been criticised particularly in view of the peculiar constitutional set-up of international
law, and it tends to distract attention from some of the more important problems by its attempt to establish a clear separation of substantive
and procedural elements, something difficult to maintain in international
law" (from M.N.Shaw)
The bottom line of my 3 posts is this: if examiner asks you about the material sources of IL, you should mention all the sources (treaties, custom, GPs, J-decisions and academic writings).

Yes. That is what made me confused. I read that chapter and it includes all the sources under material heading.