Mappings determine which cards can be used for which channels. If a channel is mapped to a card it means that card can be used to view/record the channel. Mappings are automatically created when you scan for channels but it can be useful at times to do manual mappings.
For example, if you have 2 TV cards that are the same (such as DVB-S) the first TV card might find some channels that the second might miss when scanning. This will enable you to map the missed channels found by the first TV card to the TV card that missed them.
Another situation where you would use mapping is when you manually create analog channels for Set-Top-Boxes in which case you need to manually map the new channel to an analog card.
When the "EPG Only" option is selected it means that the channel is mapped to the card but that card will only be used for EPG grabbing for that channel and won't be used for LiveTV/Recordings.
The "EPG Only" option is an advanced option useful in the following situation:
In this case you would do the following:
The result: TV-Engine will use the DVB tunning details to grab the EPG but when actually viewing/recording the channel it will only use the analog cards. This means you can get reliable EPG data direct from the provide even though you don't have a CAM that can decrypt the auudio/video.