The hobbyist might think they can run 2 displays from the PI (one digital, one analogue) at the same time, however the GPU display driver is configured solely to drive EITHER the RCA Video Out OR the HDMI (the 'switch' between the two is automatic when the driver 'senses' a HDMI cable is inserted, although you can 'force select' the output path to one or the other manually)
Since Feb 2014, when it become possible to change the GPU drivers, at least one GPU-VGA design has appeared (at 6 bits per colour channel see here, or Google 'Pi 666 VGA display', although 'dual monitor support' is still 'TBA' (as of Dec 2015)
So, if you can't use the RCA for video at the same time as the HDMI, what can you use it for ?
The answer, to the annoyance of the average hobbyist, is 'almost nothing' = the GPU pin that drives the analogue RCA socket is used for 'Video output only' and only when HDMI is not in use
If only the RCA socket could be used as a composite analogue video input, the Pi could (maybe) have supported input from older CCD cameras (common in the 'CCTV' industry, as well as used by Astronomers). Another obvious alternative use for the RCA socket would be to use it to support S/PDIF output, thus allowing AC3 or Dolby ProLogic II audio to drive the older Home Cinema systems since (as discussed later) the Stereo socket is indirectly wired to the GPIO pins via a 'low pass filter' circuit (which turns DPLII into trash). Needless to say, the actual track to the RCA socket is inaccessible from the surface of the PCB (so the Hobbyist can't even make use of the physical socket by cutting the track and wiring the socket to some other i/o pin) although it might be possible to 'drill it out' by following the trace from the 'Gerber' (PCB layout) From the Pi circuit diagram (page 2), the RCA output is connected direct to BCM2835 'DAC_TERM' pin N15, and also, via R44 (15R), to 'VID_DAC' pin P14. Diode D4 (BAV99) which protects the pin from reverse voltage (Gnd connect) and over-voltage (2.5v connect) and allows it to 'float' between 0v and (approx) 1v (the AV standard). In sort, it's wired as as analogue output, and that's that.
However the RCA socket does come into it's own when you want to use the Pi + Pi Camera as a stand-alone, remote 'CCTV' device. The RCA socket can deliver an uncompressed AV ('Audio Visual') data stream 'direct' (via the GPU) from the camera which is exactly what CCTV systems expect. It is thus rather important to discover exactly what you can get out of the RCA socket
In CONFIG.TXT you can :- Set the 'standard', SDTV_MODE 0 NTSC (default), 1 Japanese NTSC, 2 PAL, 3 Brazilian PAL (525/60 instead of 625/50) Set the aspect ratio, SDTV_ASPECT 1 4:3 (default), 2 14:9, 3 16:9 Set a flag (sdtv_disable_colourburst) to force monochrome mode 0 enable color (default), 1 disable Set the display pixel width x height framebuffer_width=1280 (default for HDMI, applies to RCA as well) framebuffer_height=720 (default for HDMI, applies to RCA as well) Set the amount of 'black space' border added to the video overscan_left= (-ve values allowed) overscan_right= (-ve values allowed) overscan_top= (-ve values allowed) overscan_bottom= (-ve values allowed)framebuffer_depth = 16 (default), 24 (has issues), 32 (needs framebuffer_ignore_alpha=1) framebuffer_ignore_alpha 0 (default), 1 (needed when using 32bit frame buffer) (It does not appear to be possible to disable 'interlace' ...)