Plugins are PHP files that can be written to extend the functionality of the Profuturus system. They are designed to provide custom intrepretation of the data in the Inventory that Profuturus itself cannot provide. An example of such would be a hyper link that looks up the data under a "User Name" column of your inventory in an online phone directory.
Plugins are kept in the "plugins" subfolder under the Profuturus root. This folder is never overwritten when updating, so barring changes to the plugin API, your plugins should keep working when you upgrade to a newer version of Profuturus.
Plugins are named by the page that they are associated with. For example, a plugin named "inventory_mod.php" will show up whenever the main "inventory_mod.php" page is called.
As of this writing, I would not suggest using plugins, as I have not developed an API for them, or finalized how they will operate.