SP518: This is the number of the start position (SP) that corresponds to the standard chess setup (RNBQKBNR). It has an entrenched, vested interest behind it, starting with the segment of the chess publishing industry that specializes in opening material.
If you're interested in the numbering of Chess960 start positions, Wikipedia has a good explanation. The Starting Position Generator is also worth a look, although getting it to work correctly takes a little trial and error. If you have trouble, the visual lists of FRC Starting Positions should help.
Substandard online resources: Almost every online resource dedicated to Chess960 that I've looked at has something wrong with it. The most common problem is that the material hasn't been updated in some time. Is this a symptom of an overall lack of interest in Chess960? The technical resources are also unreliable. For example, the 'Starting Position Generator' mentioned above, looked good at first, but turned out to be buggy; it returns the same start position for both SP786 and SP787, and doesn't recognize the valid string corresponding to SP787 (BRQKNRNB).
Lack of playing software: I haven't located any free software capable of analyzing Chess960 positions, although some commercial software, which I haven't tried, is available. The problem is to handle castling. Once both sides have castled, any chess playing software can handle the analysis. The lack of software could be a blessing in disguise. Computer analysis is largely responsible for the slow strangulation of creativity in the chess openings.