Metadata serves a number of important functions such as

The discussion about metadata initiated by librarians mainly focused on the discovery function. However, modern e-Research will require considering the other functions as equally important. New functions such as profile matching will certainly be required.

Due to the range of different functions some experts speak about different types of metadata descriptions such as structural MD, administrative MD, guide MD, preservation MD, technical MD, process MD, descriptive MD, etc. These categories are not standardized, very much dependent of the community using them and are subject to changes. In this document we will not use these terms since we did not find them helpful.

We then need to address the question how we can describe the characteristics of data so that the above-mentioned functions can be realized. The typical way is to define a number of meaningful keywords that can describe the properties of a digital object, its context and provenance. A few examples are typical keywords such as:

With each of these keywords some form of vocabulary or constraint can be associated. With respect to the category “country” for example one may want to associate the official list of nations as accepted by the UN. Such a list of possible values is called a controlled vocabulary. With “date” one may want to associate a certain form to be entered such as the US way of writing dates. Such syntactical limitations on the values are called constraints. However, for many fields such as “genre” there are no widely agreed vocabularies, i.e. one can only indicate a few typical options, but the list of values basically needs to be open.