A transclusion allows to include the content of other zettel into the current zettel.
The transclusion specification begins with three consecutive left curly bracket characters ({, U+007B) at the first position of a line and ends with three consecutive right curly bracket characters (}, U+007D). The curly brackets delimit either a zettel identifier or a searched zettel list. You can add some attributes, although a transclusion does not support the default attribute. Any other characters in this line will be ignored.
This leads to three variants of transclusions:
Transclusion of the content of another zettel into the current zettel. This is done if you specify a zettel identifier, and is called zettel transclusion.
Transclusion of the list of zettel references that satisfy a query expression. This is called query transclusion.
Transclusion of the content of an image, referenced by a hosted or based link / URL.
The first two variants are described on separate zettel:
Zettel transclusion
Query transclusion