1. The "AT" means "air tight". If I recall correctly, Title 24 requires AT and IC (insulation contact) rated cans. For most rooms, you can then either have high efficacy (approved fluorescent or LED), or can use incandescent with a dimmer. For bathroom, any incandescent has to be on an vacancy sensor (automatic off, manual on). For kitchen, most of your lights will have to be high efficacy, which also means that someone cannot pull them out and put in incandescent bulb. For fluorescent cans, you will see that they have the pin connectors. GU24 cans are NOT APPROVED for CFL, but are allowed FOR THE PRESENT for approved LEDs -- this is because the LED offerings are still developing.

2. For the kitchen, it will have to be fluorescent (tubes or pin-type socket, not CFL screw-in), or approved LED with a non-screw connection. This can be some funky vendor-specific connector, but I would steer clear of those and get GU24 cans. I put 18 of these (6" not 4") in my kitchen. For the living room, I used screw-in, even though I'm using LEDs there also -- just more versatile with more offerings.

Note the the adapters that convert a screw in to a GU24 are NOT RECOGNIZED by Title 24 -- gotta be a GU24 socket wired in, if it is something that has to pass that part of Title 24.

3. The others look different, and are available in other color temperatures than 2700K. A pretty good competitor would be the equivalent Sylvania LEDs, which are 3000k if you want it a bit whiter.

I can't remember right now, but some 4" GU24 cans (Elko?) have the socket on an adjustable bracket, and some (Nora) have it affixed to the top of the can. If the wire pigtail on the CR4 were 1/2" shorter, or I had larger hands, I would not have been able to get the CR4 installed in the Nora can. For that reason, I recommend the Elko one.

"Approved" means on the official CA list in the case of LEDs, or designed in certain common/standard ways for fluorescent.

Thanks for the quick reply attofarad (love the name too!)

Ok, so you confirmed what I was thinking. It looks like I should go with the Nora/Elco GU24 lighting at least in the kitchen. I guess Halo could put the "Title 24" label and certification on their Edison base housing because it is "Title 24" compliant everywhere but the kitchen?! IOW, other rooms simply require AT/IC housings but kitchens and bathrooms have the addition non-screw type base requirement?

The other quirky thing is that Nora doesn't mention anywhere that their 4" GU24 can is Title 24. Is there a reason for that? You would think they would want to highlight that.

Did either of you end up using the Nora or Elco 4" housing for the CR4? Were you able to pass inspection ok? We're about to put these in and I'm debating between the Nora and Elco. Thanks!