Consider this exchange:
Speaker 1: Thank you.
Speaker 2: That's all right!
In this case, speaker 2 is very casually acknowledging that speaker 1 has thanked them. It is somewhat lighthearted. This is similar to, but less polite, than an actual "welcome":
Speaker 1: Thank you.
Speaker 2: Welcome!
These are both very informal and and would not be used in a professional or business setting, or where you do not know the speaker well. It is considered much more polite to say "you're welcome" when you are thanked.
In the second case:
Speaker 1: Thank you.
Speaker 2: All right.
I would not say this, it definitely comes across as rude. In this instance, it is only an acknowledgement that speaker 1 has spoken, without speaker 2 offering any sort of welcome in return. It would be the equivalent of:
Speaker 1: Thank you.
Speaker 2: Ok.
It's always better to be polite in formal situations. I always say "you're welcome" to everyone.
Anyone: Thank you!
Me: You're welcome!
To answer the second half of your question:
When you agree someone's proposal, you would say "OK" or "All right." What is this sentence short for?
Answering someone's proposal with "OK" or "All right" generally means two things:
1) You have confirmed you understand what they are talking about.
2) You are agreeing with their proposal.
It is a colloquial shorthand for saying "I understand and I agree."
A similar question has been addressed and closed here:
https://english.stackexchange.com/questions/84837/whats-the-difference-between-hallow-sacred-holy-and-saint
I'll still take a stab at this one.
Saint is easy to separate from the other two because there are formal criteria for a religion to recognize somebody as a saint. As you said, a saint is a person. The term is used colloquially in reference to people who are really nice, as well.
Holy and sacred both imply that the thing being described is dedicated to a deity. Yes, they are most commonly used in Jewish, Christian, or Islamic writing. They don't have to be. The gate to Valhalla in Nordic Mythology is the Holy Door. Wiccans frequently refer to things as sacred.
I would say the primary difference between the two is Holy is an intrinsic trait, and sacred is an attributed trait. Think of if like Holy is an actual property of the thing being discussed. Sacred is the reputation of the thing being discussed.
See also:
http://www.differencebetween.com/difference-between-sacred-and-holy/
Best Answer
If there is a nuanced difference, true refers to the utterance (the statement is factual) and right refers to the person's understanding (your understanding is correct).
We tend not to say
but
and
And even when we say "No, that's not right" after someone says something we know to be incorrect, when we say "That's not right" the meaning is "you have gotten it wrong". We're not saying that the statement is "false" because it could actually be more complicated than a simple binary true/false. We're referring to the speaker's understanding (or misunderstanding) of something.