Use
This tag is usually used outside any loop.
So in /squelettes-dist/inclure/header.html it is called as:
<div class="clearfix header">
<h1 class="spip_logo_site">[(#LOGO_SITE_SPIP|image_reduire{224,96})]#NOM_SITE_SPIP</h1>
[<p id="slogan">(#SLOGAN_SITE_SPIP)</p>
</div>
Define site slogan
The slogan of the site is to be defined in Configuration -> Site Identity.
See about this Modifying the name of "My SPIP site"