Question on AR15 receivers

[JHansenAK47]

I was looking at aimsurplus and saw this receiver.
http://www.aimsurplus.com/product.aspx?item=F1STSL&groupid=6

Low Shelf for use with RDIAS.

I don't what low shelf means so I thought would ask some of the more enlightened people here on THR.
Just what does "Low Shelf for use with RDIAS" mean?

 

[sv51macross]

It means that the receiver has been machined with the possibility of installing a NFA-registered drop-in auto-sear. There are two methods of legally getting full-auto safely. There's buying a M-16/ect or buying a registered, pre-86 auto-sear that is designed to install in an AR-15 without having to drill a pin hole for it. (please correct me if I'm wrong). Add a safe/semi/auto trigger and a M-16 BCG, and you have a full-auto gun.

In simple terms, the Spike's Tactical lowers have the option to add a $16,000 fun-switch without additional machining. Doesn't inherently impact operation or ownability, but just an open option.

 

[JHansenAK47]

Thanks for your reply SV51macross. I googled Registered Drop in Auto sears after you told me what RDIAS stands for and found this.
http://www.quarterbore.com/nfa/dias.html