A zettel representation that is a s-expression (also known as symbolic expression).
It is (relatively) easy to parse and contains all relevant information of metadata, content or the whole zettel. For example, take a look at the Sz encoding of this page, which is available on the Info sub-page of this zettel:
/z/00001012920516?enc=sz&part=zettel,
/z/00001012920516?enc=sz&part=meta,
/z/00001012920516?enc=sz&part=content.
Some zettel provide a more detailed description of the Sz encoding.
If transferred via HTTP, the content type will be text/plain.