Why did the community make the distinction between "selection" for the Receiver of Memory and "assignment" for all other occupations?
The Elder person said that he was selected and not assigned why is that?
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.
In the book The Giver the children are relatively the same with some personality differences. This is demonstrated by the discussion of Asher having a hard time learning his words and being a kind of comical person.
Each of the children are observed throughout their life. It is common for the elders to decide on their assignments based on their personalities. Their assignments will enable them to be a part of the community and serve as productive members of the community.
The person who is to become the receiver has to be very special. The nature of his having to hold all the memories and not being able to relate to the others will alienate him from the community. Because of the strength needed in the chosen person the selection process is very different. There is more debate and careful consideration. This is why he was selected. He will stand out from the rest of the community.