What is meant by a 'high shelf' or 'low shelf' receiver?

Is one harder to build on than another?  Can a Jewell trigger be used in both?

Who makes high shelf receivers?  Who makes low shelf receivers?
Does LMT or LAR machined receivers have a high or low shelf?

Thanks in advance for any info.

All my best,
Rob

All commercial AR makers do something in the production process to inhibit the ability of an owner to insert and use a drop in auto sear(DIAS).  Colt, for instance, builds thier current models with a wall between the firing parts part of the lower, and the rear, where the lug of the upper goes when the piece is assembled. Earlier Colts had a pinned in steel block in the area.

Bushmaster, DPMS, and a couple of the samller producers leave a portion of the right hand side wall thicker, so a DIAS can't be dropped in and be centered for function.  This is clearly a joke, because all one has to do is either get a DIAS with a thinner side wall, or thin down the side wall of an existing DIAS.

Right now, the most common way to prevent use of a DIAS is to leave the bottom shelf of the rear of the lower high enough, so that there is no room for the bottom of the DIAS to fit.  These are the so called "high shelf" units. They do not interfere with the building or utility in any way, and so far as I know, they do not interefere with installation of a match trigger. The Colt wall block does, I am told.

Its just the area right below the rear takedown pin.

Continental Machine is who is doing the high shelf that I have seen... which includes RRA, Stag Arms, Century, to name a few.

It really wont make a difference for typical use and building, but if you ever planned to use an accuwedge, forget about it.... cause the shelf keeps you from doing it.

Quoted: It really wont make a difference for typical use and building, but if you ever planned to use an accuwedge, forget about it.... cause the shelf keeps you from doing it.

No it doesn't. You just have to cut the bottom of the AccuWedge to about half it's original thickness

Quoted: All commercial AR makers do something in the production process to inhibit the ability of an owner to insert and use a drop in auto sear(DIAS).  Colt, for instance, builds thier current models with a wall between the firing parts part of the lower, and the rear, where the lug of the upper goes when the piece is assembled. Earlier Colts had a pinned in steel block in the area. Bushmaster, DPMS, and a couple of the samller producers leave a portion of the right hand side wall thicker, so a DIAS can't be dropped in and be centered for function.  This is clearly a joke, because all one has to do is either get a DIAS with a thinner side wall, or thin down the side wall of an existing DIAS. Right now, the most common way to prevent use of a DIAS is to leave the bottom shelf of the rear of the lower high enough, so that there is no room for the bottom of the DIAS to fit.  These are the so called "high shelf" units. They do not interfere with the building or utility in any way, and so far as I know, they do not interefere with installation of a match trigger. The Colt wall block does, I am told.

Bushmaster lowers are DIAS ready and so are Sendras, A1 style Eagle Arms, and A1 style PWAs.

Forged DPMS lowers are also "low shelf"