Many frameworks consist of boxes with arrows drawn between them. When people introduce a framework, they usually start explaining what each arrow means, e.g. how A -> B represents the specific ways in which A influences B, going down a list of possible interactions.

There is  a different way of looking at the information which the framework provides, which is not what's in the arrows which are drawn, but rather in the fact that some arrows are missing. Every missing arrow is an assumption that the two factors concerned do not affect each other directly, and these are the assumptions which simplify and accelerate understanding.

Frameworks where every possible arrow has been drawn are uninformative in this regard, and can be replaced by a simple list.

Wikipedia:

In simplified terms, the theory of the firm aims to answer these questions^[1]:

1. Existence - why do firms emerge, why are not all transactions in the economy mediated over the market?
2. Boundaries - why the boundary between firms and the market is located exactly there? Which transactions are performed internally and which are negotiated on the market?
3. Organization - why are firms structured in such specific way? What is the interplay of formal and informal relationships?

Part of question 2: the boundary between firm and market is sharp iff firm-firm interactions can be characterized by a few simple dimensions like price and quantity - we call these transaction-based interactions, and call such products commodities. When we don't know how to reduce the interaction, the idea of a "market" is not useful, and we call those interactions relationship-based.

It's a false spectrum, because "transaction-based" is a lot more informative than "relationship-based", which really means "everything else" - we say "relationship-based", but I think it is more aptly described as "non-transaction-based".

I met a college friend, an economist who works at a REIT, for lunch. The discussion turned to Case-Shiller ETFs, how there aren't any. I speculated that the difficulty of bridging the huge liquidity gap between an ETF and residential real estate has something to do with it. In the case of the paired funds that MacroShares was slated to launch but never did, maybe the amount of buffer needed would have been too big.

Basically, the arbitrage mechanism behind a Case-Shiller index would have such high liquidity premium that it would hardly track the real thing.

Unverified, but related: my roommate has informed me that the GLD ETF is currently worth a lot more than its contractual gold equivalent.