When talking about Plesk server preferences, we mean the following:
Plesk server preferences are held by the prefs node. The node is presented by complex type serverPrefs (plesk_server.xsd) and structured as follows:

XML request packets setting up or changing server preferences may look as follows:
<packet version="1.4.2.0">
<server>
<set>
<prefs>
<stat_ttl>3</stat_ttl>
<traffic_accounting>3</traffic_accounting>
<restart_apache_interval>0</restart_apache_interval>
</prefs>
</set>
</server>
</packet>
This packet is formed for sending to the server running Plesk for Unix. When operation succeeds, both inbound and outbound traffic will be included to server traffic statistics, which will be stored during 3 months. Also, Apache will not be restarted automatically.
<packet version="1.4.2.0">
<server>
<set>
<prefs>
<stat_ttl>5</stat_ttl>
<traffic_accounting>2</traffic_accounting>
</prefs>
</set>
</server>
</packet>
This packet is formed for sending to the server running Plesk for Windows, only outbound traffic should be calculated, and the traffic statistics should be stored during 5 months.
XML response packet retrieving server preferences may look as follows:
<packet version="1.4.2.0">
<server>
<get>
<result>
<status>ok</status>
<prefs>
<stat_ttl>4</stat_ttl>
<traffic_accounting>3</traffic_accounting>
<restart_apache_interval>86400</restart_apache_interval>
</prefs>
</result>
</get>
</server>
</packet>
This packet is received from the server running Plesk for Unix where both inbound and outbound traffic are included to server traffic statistics, which is stored during 4 months. Also, Apache is restarted each 24 hours.
<packet version="1.4.2.0">
<server>
<get>
<result>
<status>ok</status>
<prefs>
<stat_ttl>6</stat_ttl>
<traffic_accounting>2</traffic_accounting>
</prefs>
</result>
</get>
</server>
</packet>
This packet is received from the server running Plesk for Windows where only outbound traffic is calculated, and the traffic statistics are stored during 6 months.