I think in the context of your study today on identify theft, metadata is an element, but a small element, of what is used to construct an identity. If someone is trying to engage in identity theft, there are very specific pieces of information that they need to know about an individual or need to create concerning an identify in order to create a viable, semi-functioning cyber organism.
Metadata itself is more about the transactions. It's more about your interests. It's more about the transmission of the data. In particular contexts, that can be extremely relevant, as in the case of IP addresses, but it is less relevant in terms of your search preferences, your search results, maybe even your location history. The reason you have heard many different definitions of it is that within the context of whom you're speaking to and within the way it is applied, it can have very many different and many constructive applications.