Circling back to properties, when you set a value on an attribute or metadatum or author a relationship within a layer, you are effectively expressing an opinion on that value. This is named an opinion because value assignments can always be changed by others! This is a very important concept for composition.

A different way of looking at this would be that “setting a value” implies that that action is final and the value itself becomes immutable; whereas with opinions you merely mean

“Within this layer, I want the value for this to be <INSERT VALUE>

If the value already existed before, you are essentially “overriding” it