• Standard
This will record to and play back from the card that is set in the record/play menu setting (see 2 below). If there is only one card fitted it is that card; if two are fitted it can default to the first one inserted when the cards are put in if the camera is left on or one card has been removed.
It doesn't matter whether the camera is set to record RAW, JPEG or both – whatever file format the camera is set to record will be to that nominated card.
• Auto switch card (Not available on EOS-1D Mark II/N or 1Ds Mark II)
When this setting is active the camera will record and play back to the nominated card, so long as there is space on the card. If that card becomes full it will seamlessly switch to using the other card. In the days when the cards were a lot smaller this was useful; today with 64GB and 128GB cards available it is rare to fill the main card up.
• Rec. Separately (Not available on EOS-1D Mark II/N or 1Ds Mark II)
This option is only relevant if you shoot more than one file format, i.e. if you are shooting both RAW and JPEG and have two cards fitted. This allows you to record the RAW files on one card and the JPEG files on the other.
To set which format records to each card, go to the Q screen, then select each card in turn, choosing the preferred file format. This is useful if, for example, you normally shoot JPEG, but like to have a RAW file just in case. One downside with this is that you have two cards to download if you intend to keep everything you shoot.
• Rec. to multiple
This is the belt-and-braces approach. This option records whatever file formats you've selected to both cards, giving you two identical copies of each file, if you then download both cards. On the plus side, it gives reassurance if you're concerned that a card might fail. However, you need a good stock of media cards and time on your hands to manage the file duplication successfully.