Digital TV signals usually have multiple channels on one frequency. This means you can tune into all of these channels at the same time using a single TV tuner card.
RikPVR is adding support for multichannelling so that the same card will be used to record any shows which are on the same frequency. e.g. If you record shows on Channel 7 and 7HD at the same time it will only use one card for both of these recordings.
The multichannelling feature is currently in Alpha testing (as at Nov 4, 2007) and should be released for public testing soon.