"3rd party" means "not supplied with Vegas".
SCS does not support (as in technical support) ANY 3rd party codec, and this option is checked by default.
This door has to be opened by YOU the user, but if this door was not here, I doubt Vegas would be making the gains it has.
This single check box eliminates all codec hassles and imparts true freedom from the chains.
This knowledge is so valuable that it puts an end to paying huge fees for plug-ins, upshifters, books, seminars, and "demystifies" the matter.
Making money from newbies is a giant industry, and there's no motivation to enlighten anyone because it's counter-productive.