It means if you have a fully pumped or a gravity hot water system. You should be able to tell if you have a gravity system by the looking at the pipes coming off of the back boiler, if there are only 2 then you have a fully pumped system, if 4 then probaly gravity. The other way to tell is if with your old timer you could have h/w only or c/h & h/w. Then it's prob gravity. If you can have c/h on it's own then it's fully pumped.
howtomendit.com for free repair help, information and advice.
Try fixfault.com to fix home heating systems.