relation noun in french?

Your confusion actually comes from that fact that you are learning French through English which uses the same word for at least four different concepts.

"That" can be a demonstrative adjective. "That bird is ugly" (Cet oiseau est moche). It's an adjective so it's always followed by the noun it's modifying. If you see "that" followed by a noun and you know you could replace "that" by "this", then it's a demonstrative adjective and translates to ce/cet/cette/ces (depening on the gender and number of the demonstrated object).

"That" can be a demonstrative pronoun. "That's ugly" (C'est moche), "That stinks" (Ça pue). It's a pronoun so it has the value of a full noun. If you see "that" and you know it can be replaced by "this", "it" or any noun, then it's a demonstrative pronoun and translates to "ça" (or in the formal forms "ceci" or "cela"). The exception is when it's the subject of the verb "être", it has to be "ce".

"That" can be an adverb. "I'm not that ugly" (Je ne suis pas si moche). If you can replace "that" by "that much" or "that many", then it's an adverb and translates to "si".

"That" can be a relative pronoun "The bird that I heard talk" (L'oiseau que j'ai entendu parler <= que = l'oiseau (object), "J'ai entendu parler l'oiseau"), "The bird that talked" (L'oiseau qui a parlé <= qui = l'oiseau (subject), "L'oiseau a parlé), "The bird that I heard talk about" (L'oiseau dont j'ai entendu parler <= dont = de l'oiseau (object), "J'ai entendu parler de l'oiseau").
It's a pronoun linking two phrases in a sentence and represents the previous phrase in the new one. In English, it can often be omitted ("The bird that I saw" can be "The bird I saw"). They are translated as either "que", "qui" or "dont".
If the relative pronoun is the object of the verb in the new phrase, it's "que". If it's the object of the new phrase and represents something that starts with "de", it's "dont". And if it's the subject of the new phrase, it's "qui".

In your first example, "Être patron d'une grande entreprise, c'est une lourde responsabilité" would normally be translated to "Being the boss of a big company is a big responsibility" but literally means "Being the boss of a big company, it's a big responsibility".

In the second example, "Le déplacement de l'entreprise en province, c'est quelque chose à quoi il faudrait refléchir" would normally be translated to "The moving if the company outside of the capital is something we/one should think about" but literally means "The moving if the company outside of the capital, it's something we/one should think about".

It's a sentence structure that is relatively uncommon in English but extremely common in French. Separating a part of the sentence from the rest of the sentence into two different phrases is used to stress stress that part. It announces that this first part you separated and placed first is the topic of the sentence.

So in both sentences, "c'est" means "it is" (that is, this is). It's a demonstrative pronoun.

Really your problem is that you're learning French through the English language.

"that" has many different meanings in English, most of which are translated by different words.

1. ce = this or that, as a subject pronoun (mostly when the main verb is être). ce - also appears in other constructions. It's demonstrative, meaning "over there/here". That's the use in your example sentence. C'est un homme = This/That is a man. (It's also said instead of saying: He is a man. Il est un homme -- is rare in French; when a noun is the subject complement, ce is used instead of il/elle.

note: ce, can also be plural in the same way. = these/those. Ce sont des étudiants. Those/These/They are college students.

2. ce = this or that, as a demonstrative determiner, for masculine singular nouns, when the first sound of the word that follows is a vowel. A noun must follow. ce chien = this dog OR that dog.

3. que = that/which/whom; as a relative pronoun introducing a dependent clause, where it functions as the direct object of the dependent clause. C'est le livre que tu m'as offert. That's the book (that/which) you gave me. que, in this use, is often optional in English but is required in French. But as a subject, you use "qui" instead. C'est le livre qui m'intéresse. That's the book that/which interests me.

4. que = that, as a conjunction, introducing a dependent clause used as a noun. Je crois qu'il a raison. I think (that) he's right. that - is optional in English, but required in French.

5. que - has a ton of other uses: whether, so that, then, introducing third person commands, to replace other previously stated conjunctions, negative adverb, interrogative pronoun, exclamatory adverb, part of a conjunctive phrase, than.

6. Here is nice phrase that uses "que" and "ce", in multiple ways:

Qu'est-ce que c'est que ça ? What is that?

literally: What (interrogative adverb) is it (this/that) which (relative pronoun) that/this (subject pronoun followed by "être") is [another que - not easily translated, it connects ça to the rest of the sentence) ça - that (not followed by être). que ça - is an unnecessary repetition of ce (in c'est), in order to emphasize it.

Qu'est-ce que c'est ? = What is it? adding on, "que ça" = What is THAT?

note: ça is in the informal abbreviation of "cela" (that). ce= this/that; là = there. together, cela = that there (that), as a demonstrative pronoun when not followed by "être" as a main verb.

Glad you asked?

7. your update:

: Le deplacement del' entreprise en province,c'est quelquel chose a quou il faudrait relechiir -- many mistakes. The correct version =

Le déplacement de l'entreprise en province, c'est quelque chose à quoi il faudrait réféchir.

Best English translation = Moving the company to the provinces is something to be reconsidered.

quelque chose = something. quelque = some; chose = thing. quelque comes from quel (which) + que (ever - in this use) . whichever thing - literally, but means "something".

quelque chose à quoi il faudrait réféchir. - literally = something about which it would be necessary to reflect. (something which must be reconsidered).

what/which/that/who/whom do not translate nicely to French in a one-to-one way.

Ce . it's
Que, that

Quelque chose = some thing (something)

From me as a native French person, C' and ce are demonstrative pronouns.
In the context you mentioned on here: '' C'est une lourde responsabilité '', c' est " is like: '' ce est ''. But you can't write: ce est, it's incorrect in French.

These pronouns can replace a nominal group or an entire sentence, examples:

_ Je viens de perdre mon travail. C'est très embêtant. / I just lost my job. It's very annoying.
- C'est triste qu'elle soit malade / It's sad that she's sick. Here again: '' C'est triste .. '' = Cela est.

• Elision: like above: c' generally consists of replacing with an apostrophe one of the final vowels "a, e, i" of a word, when it is followed by another one beginning with a vowel "a, e, i, o, u".
or beginning with a silent " h ": The deleted vowel is called "elidee" and replaced in writing by an apostrophe.

_ Es-tu allé au concert hier ? / Did you go to the concert yesterday?
- Oui et ce que j'ai écouté m'a beaucoup plu / Yes, and I really liked what I listened to.
In this case, ' ce ' replaces : La musique / the music. Thus the person who talks replaced the word musique by ' ce'.
__________________________________________________
_ QUE
Que replaces the complement and is followed by a topic. Example:
- Tu as acheté un chien. (You bought a dog.) → Subject + verb + complement
Le chien q̲u̲e̲ tu as acheté (The dog which you bought)