The #DOSSIER_SQUELETTE tag such as the #CHEMIN tag is used to develop easily transportable and exchangeable templates.
The #CHEMIN tag, which searches for a file in all the templates, plugins or SPIP should be preferred most of the time.
#DOSSIER_SQUELETTE tag  returns the path of the directory in which the tag is used. Just like #SQUELETTE returns the name of the file.
Note: these tags are therefore quite similar to the constants __DIR__ and __FILE__ in PHP.
Example:
This allows you to call the files attached to the template in the template directory: for example with this structure
-  
article.html -  
css/my_style.css 
We can write in the template article.html :
<link rel="stylesheet" href="#DOSSIER_SQUELETTE/css/my_style.css" type="text/css" />
This will load the css/mon_style.css related to article.html.
Prefer #PATH:
In most cases, however, the #CHEMIN tag will be preferred in this way, which facilitates possible overloading of files:
<link rel="stylesheet" href="#CHEMIN{css/mon_style.css}" type="text/css" />