An important aspect of knowledge work is to interconnect your zettel as well as provide links to (external) material.
There are several kinds of references that are allowed in Zettelmarkup:
Links to other zettel or to (external) material
Embedded zettel or (external) material (inline transclusion)
Reference via a footnote
Reference via a citation key
Put a mark within your zettel that you can reference later with a link.