Why did the community in The Giver make the distinction between "selection" for the Receiver of Memories and the "assignment" for all other occupations?
For every other job in the community, the Committee of Elders could look at characteristics such as demonstrated interests, physical strength, or the needs of the community for people to fulfill certain jobs. These were all fairly straight-forward criteria; the Committee of Elders could evaluate an individual and assign an occupation without undue concern that problems might arise from a less-than-ideal assignment.
The Receiver of Memories, on the other hand, preserves all the history and outside experience that the rest of the community no longer possesses. When the Committee of Elders is faced with making a decision that requires reflection and consideration in the light of those learnings, they rely upon the Receiver of Memories to provide them with the guidance and knowledge that will enable them to make the correct decision for the community. As the Giver explained to Jonas,
There's much more. There's all that goes beyond – all ... that is Elsewhere – and all that goes back, and back, and back. I received all of those, when I was selected. And here in this room, all alone, I re-experience them again and again. It is how wisdom comes. And how we shape our future.
Recognizing the vital importance of the Receiver's role in the preservation of the community, a new Receiver was very carefully selected. The chosen individual must have be honest, intelligent, courageous, capable of learning much more, and must have "Capacity to See Beyond" - the community's way of describing the ability to recognize emotions and characteristics and situations that are lost to the community but essential for the Receiver.