Totally irrelevant to meaning. With human head nouns it's a free choice between wh-relatives and that-relatives: "one of society's masses who enjoys helping people" and "one of society's masses that enjoys helping people" show no semantic differences, and no syntactic differences other than what follows from "that" not being a pronoun ("one of society's masses to whom the letter was sent" is grammatical but "one of society's masses to that the letter was sent" is of course not).
That advice is based on the assumption that your example is of a restrictive relative clause (the kind that doesn't need a comma) which it seems to be. But in constructions containing a non-restrictive relative clause (the kind where a comma is required), the situation is different. This might help you spot the difference:
[1] "Politicians who/that make extravagant promises aren't trusted". [restrictive]
[2] "Politicians, who make extravagant promises, aren't trusted". [non-restrictive]
In [1] the relative clause in bold is of the restrictive kind, that is to say it restricts our understanding of which politicians are being referred to. It's an integral part of the larger message. It's not saying that the lack of trust applies to all politicians, but just the ones who make extravagant promises, hence its name 'restrictive', and the absence of a comma.
In [2], by contrast, the property of not being trusted applies to all politicians in general. It doesn't pick out any particular group of them, like [1] does, hence its name 'non-restrictive'. Think of the non-restrictive kind as providing information that is supplemental to that expressed in the rest of the sentence. It's irrelevant to our understanding that politicians in general aren't trusted. Which is why it's set of with commas (or sometimes dashes).
In the case of restrictive relatives like [1], it is perfectly okay to use "who" or "that", which is what I meant be 'free choice'. But with non-restrictive ones (the kind with a comma) like [2], "that" is not normally permitted and you should stick with "who".
(Incidentally, the words "as" and "the" in "as one of the society's masses" look dodgy to me. Best get rid of them!)
Where should be used here; the focus is on the event (the 2002 World Junior Championships), rather than the time period. Had the event and location been omitted, and only the time given (“Bolt took his first shot at success in 2002, ...”), when would be used, as the focus now becomes the time period.
Best Answer
No, that "for which" is wrong." At a minimum, it should be "for which reason."
However, it's better to have it changed, e.g., to:
As presented during the seminar, the development of public health terminologies and classification is of prime importance to us. This is why it is necessary that we all collaborate towards our collective mission to make a difference in the lives of people of the world.
or:
As presented during the seminar, the development of public health terminologies and classification is of prime importance to us. This is why it is necessary that we all collaborate towards our collective mission of making a difference in the lives of people of the world.
I prefer "mission of making a", but "mission to make a" is more frequent at Google Books.
The comma is important too.