trilium/doc/0001-Welcome to Trilium!/0001-Note cloning.html

Note cloning allows you to "register" single note (and it's subtree) under
multiple different notes. 
<p>&nbsp;</p>
<p>This allows the same note to appear in multiple places in the tree. This
  is useful when note logically belongs to multiple categories - e.g. in
  time based structure you can categorize the note for the given date and
  then separately you might want to put it somewhere else by it's taxonomy.
  In this sense it works similarly to tags, only more generally and in a
  tree structure (while tags are typically flat structured).</p>
<p>&nbsp;</p>
<p>When note has multiple parents, then technically there isn't any one "original"
  and "clones", all of the parents are equal.</p>