This is most often applied to television, where digital TV (DTV) channels are in-band adjacent-channel to analog ones. Channel 8 might use channel 32 for its ATSC or DVB transmission, but a virtual channel map or virtual channel table (VCT) allows viewers to tune to channel 8 on a digital set anyhow. DTV tuners typically scan all channels for this information while they are turned "off", and store it in memory for instant access later on.
Because DTV can carry multiple programs simultaneously, virtual channels also map out subchannels. Typically in the United States and Canada, the main program is on channel 8 (or 8.1), in the example if the station does not have more than one digital subchannel. Other programs are on 8.2, 8.3, and so forth up to 99, with data channels from 100 to 999. In this case, 8 is the "major channel", and 1, 2, or 3 is the subchannel. There is no practical difference between a dot and a dash, though the dot looks familiar to FM listeners, and avoids confusion with a range (for example, 2.4 is the multiplex 4 of channel 2, not 2 to 4). A dot is also easier to display on LEDs in set-top boxes as it does not occupy a full digit position.
Australia has a slightly different format due to the smaller number of main channels and doesn't require a dot or a dash (for example, ABC2 22 is a digital subchannel of 2, ABC1), while the use of multiplexes throughout Europe, Africa and the Middle East leads to subchannels being treated like main channels (e.g. in the United Kingdom, ITV2 has its own channel number, 6, distinct from ITV1's channel number 3)
Virtual channels are also used on direct broadcast satellites, such as Dish Network, DirecTV, and Astra. Rather than a few dozen channels with a few subchannels each, these services map to a range of hundreds of individually numbered channels. Cable TV also does this on digital cable, as does satellite radio.
Digital radio also uses channels and subchannels, but only for DAB. The IBOC systems (iBiquity and Digital Radio Mondiale) do not currently use any virtual channels because of the limited bandwidth available in analog sidebands. As IBOC progresses and bandwidth improves due to new technology and the discontinuation of stereo subcarriers (and later analog althogether), virtual channels may take hold there as well.