A zettel transclusion is specified by the following sequence, starting at the first position in a line: {{{zettel-identifier}}}.
When evaluated, the referenced zettel is read. If it contains some transclusions itself, these will be expanded, recursively. When a recursion is detected, expansion does not take place. Instead an error message replaces the transclude specification.
An error message is also given, if the zettel cannot be read or if too many transclusions are made. The maximum number of transclusion can be controlled by setting the value max-transclusions of the runtime configuration zettel.
If everything went well, the referenced, expanded zettel will replace the transclusion element.
For example, to include the text of the Zettel titled Zettel identifier, just specify its identifier 00001006050000 in the transclude element:
{{{00001006050000}}}
This will result in:

Please note: if the referenced zettel is changed, all transclusions will also change.
This allows, for example, to create a bigger document just by transcluding smaller zettel.
In addition, if a zettel z transcludes a zettel t, but the current user is not allowed to view zettel t (but zettel z), then the transclusion will not take place. To the current user, it seems that there was no transclusion in zettel z. This allows to create a zettel with content that seems to be changed, depending on the authorization of the current user.

Any attributes added to the transclusion will set/overwrite the appropriate metadata of the included zettel. Of course, this applies only to thoes attribtues, which have a valid name for a metadata key. This allows to control the evaluation of the included zettel, especially for zettel containing a diagram description.
See also
Inline-mode transclusion does not work at the paragraph / block level, but is used for inline-structured elements.