In one unpublished draft paper that I wrote a long time ago, I found that in spite of surface similarities relative and appositive clauses can be distinguished at three major syntactic levels:

(i) the nature of the head noun,

(ii) the complementizer optionality or compulsory nature, and

(iii) what can be done with the clause.

Here is the draft in question, which I hope you will find interesting and open the way for you for more insightful distinctions:

The difference between the two is rooted on both structural and meaning level:

(a) His suggestion that Mike cheated during the exam was rejected.

(b) His suggestion that surprised our teacher was rejected.

(a) - nominal appositive clause

(b) - relative clause

Structurally, in terms of the introductory word, that in (b) is a relative pronoun and could be replaced by which. It acts as the subject of the relative clause.

In (a), that is a conjunction and it does not act as a clause element (subject, object, etc.) in the clause structure.

In terms of meaning, the nominal appositive clause renames the head noun it post-modifies:

What was the suggestion? The suggestion was that Mike cheated during the exam.

The head noun suggestion is restated, and thus specified by means of the appositive clause.

On the other hand, the relative clause rather identifies the head noun it post-modifies:

Which suggestion was rejected? The one that surprised our teacher.

The relative clause points out which specific suggestion the sentence refers to as if there were numerous suggestions to choose from.

The difference is also in that the head of the noun phrase of an appositive clause must be an abstract noun such as fact, idea, reply, answer, appeal, promise, etc. Only such nouns can be restated and thus specified by an appositive clause.

The head noun and its attribute itself may be filled by various grammatical unit such as adjective, noun, or a relative clause.

A relative clause is a clause which modifies a noun. The head noun which is a word, and a relative clause are all grammatical units which have similar functions but are uniquely different in terms of structure and meaning.

For example

A. The President, Chin Chag, studied in China.

B. The President, who studied in China, is called Chin Chag.

Differences

In sentence A word is in apposition, in sentence B a whole clause is in apposition.

In sentence A there is general information but in sentence B information is explicit.

which is to give more clear distinction or explanation about something or someone that is stated in the head noun. So, what is the difference between them? Would anyone kindly leave a respond for me? Thank you in advance.

This paper by Mark De Vries gives a nice overview of the adjunct/complement distinction plus a new idea: https://benjamins.com/catalog/avt.17.21vri/fulltext/avt.17.21vri.pdf