Why did the community make the distinction between "selection" for the Receiver of Memory and "assignment" for all other occupations?The Elder...

We are not actually told why this is, but my assumption is that there are two things going on here.

First, I think that it is just for the sake of being different.  The post of Receiver is so different that it deserves a different term than the one applied to all the other occupations.

Second, the word "selection" has the word "select" in it.  When we talk about someone being part of a "select few" for instance, we are saying that that person is special.  So it sounds more special than the other occupations.