The role key defines what kind of zettel you are writing. You are free to define your own roles. It is allowed to set an empty value or to omit the role.
Some roles are defined for technical reasons:
A zettel that contains some configuration data for the Zettelstore. Most prominent is 00000000000100, as described in 00001004020000.
All zettel that document the inner workings of the Zettelstore software. This role is only used in this specific Zettelstore.
A zettel with the role
tag and a title, which names a tag, is treated as a tag zettel. Basically, tag zettel describe the tag, and form a hierarchiy of meta-tags.
If you adhere to the process outlined by Niklas Luhmann, a zettel could have one of the following three roles:
A small note, to remember something. Notes are not real zettel, they just help to create a real zettel. Think of them as Post-it notes.
Contains some remarks about a book, a paper, a web page, etc. You should add a citation key for citing it.
A real zettel that contains your own thoughts.
However, you are free to define additional roles, e.g. material for literature that is web-based only, slide for presentation slides, paper for the text of a scientific paper, project to define a project, ...