The source is how the master tape was made. A Soundboard (SBD) is a tape made from the direct outputs of the soundboard at a show. The sound is usually up front and very clear. Depending on how it is mixed there is usually little to no crowd/hall ambiance mixed in. These are generally the most desirable tapes. However, keep in mind that not all SBD master tapes are perfect and there are several examples of defects on them. In very small venues there can often be a very off balance mix if a tape is made from just the soundboard. A tape made with a mix of soundboard feed and mics used for ambiance can be very nice but done wrong can be really horrible. In the past couple years there have been some Dead tapes circulating labeled as SBDs but are actually tapes made from one of monitor mixes. These usually don't sound very good and will have an odd balance. They also might have the band talking to each other through their internal intercom system.
Audience (AUD) tapes are made with microphones in the venue. With Dead shows most of these come from the taper's section which is behind the soundboard. Front of Board (FOB) tapes are recordings that are made from somewhere between the stage and the soundboard, usually stealth. Most consider the front of the soundboard a better place to record from. Stealth recordings are recordings that are made at shows or in parts of a venue where taping is not allowed. AUD recordings are rarely as clean as SBDs, but some people prefer them because they give a better sense of "being there." As some tapers are getting into better and better equipment, AUD recording are getting better and some sound very fine. One way to identify AUD tapes is if you hear people in the audience that sound near and distinct. The crowd (if any) on SBD recordings will always sound more distant.
Recordings made off of the radio are usually designated FM. With a good tuner, FM recordings can sound quite good. Due to the limitations of radio the very highest frequencies are lacking and the dynamic range is compressed, however. Some people make finer distinctions regarding sources but that is beyond the scope of this article. If you are not sure of the source of your tape you can try to make an educated guess. It is a good idea to put a question mark on your list if you are guessing. Example: SBD?