From here you can choose which BitTorrent client to use from the available list.
rTorrent is the preferred and fully supported one, but you can also choose from Deluge and uTorrent.
rTorrent has a few versions you can choose from by clicking on the drop box, highlighting the required version and clicking Change version. After this you will have to go to Services and restart rTorrent.
There are a variety of WUI you can choose to communicate with rTorrent and they are accessed from the bottom line of buttons. The most commonly used is ruTorrent which is developed exclusively for Xirvik by the developers and has many features you will not find on any other server providers box. Additionally, you can choose wTorrent or rtGUI.