This tag is used to check for the existence of a plugin activated on the site, or to access all of the data stored in its plugin.xml file (no information is returned if the plugin has not been activated).
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#PLUGIN{plugin_prefix}returnstrueif the plugin has been activated. -  
#PLUGIN{plugin_prefix,requested_data}returns the requested data from the plugin, such as its version number, its name, its description etc. of the plugin if it is active. -  
#PLUGIN{plugin_prefix, tout}returns all of the data from the plugin as an associative array that can be assigned to another variable so that you can display it using the |table_valeur filter. 
The plugin prefix is the field entered the <prefix> XML field in the paquet.xml file which is always present at the root directory that contains the plugin’s scripts.
The most common use of the #PLUGIN tag consists of testing to see if the plugin has been activated or not before using an HTML code:
[(#PLUGIN{plugin_prefix}|oui) blablabla...]
will display "blablabla..." if the plugin_prefix plugin has been activated.
In php, the presence of the plug-in is tested using :
test_plugin_actif('prefix_plugin')