&lt;b&gt;Per Install&lt;/b&gt; should be used when you want to license your script to a single account on a server. It will use the account domain, IP address and install path based on $_SERVER variables.&lt;br /&gt;&lt;br /&gt;&lt;b&gt;Per Site&lt;/b&gt; is the same as &lt;i&gt;Per Install&lt;/i&gt; except it does not record the install path.&lt;br /&gt;&lt;br /&gt;&lt;b&gt;Per Server&lt;/b&gt; should be used when you want to license your script to a single server allowing many installations on that server. It will use the server name, server IP address and the servers Media Access Control (MAC) address. &lt;i&gt;Per Server&lt;/i&gt; currently does not work with Windows servers.