RELATION
Perhaps property is indeed currently the closest term for anything that relate, but the potential of relation is way higher; and in many case property is not the appropriate word. For example, can you imagine describing your relative being a property of you..
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Property is a static component; Relation is a dynamic component.
Feature requests like collection specific relation and limiting scope of relation rely on the concept of relation being a changeable, situational, contextual unit.
Property has a definitive hierarchy to one single object; relation enables multi-dimension and many-to-many usage. This also provides the bases for reusability.
SET AND COLLECTION
Agree on set problem. After reading, explaining on the forums, and using them, I still can’t grab hold onto the meaning of these two words, even though I understand the functions.
TAG
I don’t use tag much because it always confused me what I am precisely doing. But there are probably different approaches to tag:
Understanding 1:
In the language of graph, area of nodes, instead of property and relation. It collects items together as nearby addressing matters. e.g. #work ← This is probably why people expect collection to work like query.
However, this understanding could interrupt the concept of spaces.
Understanding 2:
As theme of contents, e.g. #PKM. If it is a context of fixed hierarchy / taxonomy, this tag should pull out all contents below. ← This adds the difficulties to implementation of richer relation.
Understanding 3:
As related / similar contents, e.g. #anytype #notion, This is an indication of sibling hierarchy / relation. ← this is an example to why relation aren’t property.
I don’t know: author is a tag; object type (e.g. book) is a tag; keyword is a tag; topic is a tag.. perhaps that’s why tana called it supertag 
. For those of you who uses tag a lot, please add your usage and clarify what you need for it to work 
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P.S. I will also find terminology and concepts difficult if I am to translate Anytype
P.S.2 May Anytype pass through the terminology struggles soon and develop something outstanding 