{profondeur=3}: allows you to select the sections on the 3rd level (sector section types are level 0, then the sections underneath are level 1 etc.).
A RUBRIQUES loop with the criterion  {profondeur=0} will give the same result as {racine} or {id_parent=0}
An example of this use is, in a multilingual site organized by sectors (each language corresponds to a root section of the site), to create a navigation menu:
<BOUCLE_example(RUBRIQUES){lang}{profondeur=1}>
<a href="#URL_RUBRIQUE">#TITRE</a>
</BOUCLE_example>
With the tree structure below:
-  Root section A in French
- Sub-section A1
 - Sub-section A2
 
 -  Root B in English
- Sub-section B1
 - Sub-section B2
 
 
The Root section A is a sector, so has a depth of 0. Sub-section A1 corresponds to depth 1.
The example loop will therefore return, if we are on an English page:
-  Sub-section B1
-  Sub-section B2