Metadata is "data about data". If you stay long with PDM industry, you probably remember how earlier EDM/PDM software defined their role by managing of "data about CAD files" (metadata). However, it was long time ago. Wikipedia article defines two types of metadata – structural and descriptive. Here is a quote from the article:
The term is ambiguous, as it is used for two fundamentally different concepts (types). Structural metadata is about the design and specification of data structures and is more properly called "data about the containers of data"; descriptive metadata, on the other hand, is about individual instances of application data, the data content.
In my view, CAD/PDM/PLM is using both types. Since design is very structured and contains lots of rich semantic relations, metadata about CAD files stored in PDM system is structured. At the same time, descriptive metadata such as file attributes, information about people, project, organization can be applied to individual instance of CAD data (files) as well.
Since early EDM/PDM days, lots of changes happened in the definition and usage of a word metadata. Some of them are very confusing. The traditional use and definition of files (for example, in context of CAD files) is changing. Sometimes, we want to to keep "file" as a well-known abstraction, but underlining meaning is completely different and point more on "data" or "design" rather than actual files. Also, introduction of web based systems are changing physical use of files. The usage of file accessed via mobile application located in Dropbox is completely different. In many scenarios you will never get access to these physical files.
DBMS2 article Confusion about Metadata speaks about some additional aspects of metadata management that getting more relevant these days. It includes data about mobile devices usage (telephone metadata) and document data. Document data is getting more structured these days and often cannot be distinguished from structured RDBMS data. Here is interesting passage that describes the transformation of database and document based data.
[data about data structure] has a counter-intuitive consequence — all common terminology notwithstanding, relational data is less structured than document data. Reasons include: Relational databases usually just hold strings — or maybe numbers — with structural information being held elsewhere. Some document databases store structural metadata right with the document data itself. Some document databases store data in the form of (name, value) pairs. In some cases additional structure is imposed by naming conventions. Actual text documents carry the structure imposed by grammar and syntax.
Modern data management systems and especially noSQL data bases such as document and key-value databases can introduce new types of metadata or data. IoT (Internet of things) brings another level of complexity to data management. I can see many others to come.
What is my conclusion? I think, the term meta-data is getting outdated at least in the scope of design and engineering. Originally used a lot in EDM/PDM systems managing metadata about CAD files is not relevant anymore. Design data originally stored in CAD files becomes more transparent and connected to external to design world. The whole data paradigm is changing. Just my thoughts…