I am not sure what your builder meant by closed, but we are also in the SE, and our custom builder has started putting sealed crawl spaces in all of their homes. I have to say...it is wonderful! The crawlspace is conditioned air...we could use it for storage! Our builder said it also helps keep the floorboards from shrinking and cupping due to changes in humidity...and I was absolutely amazed at the lack of shrinkage in our wood trim this winter. In our previous house, that first winter we had TONS of cracks and gaps in the trim...everything had to be caulked. In this house, almost nothing! It is so clean under the house, too. I would recommend it to everyone, especially in our climate.

Most building codes require a crawl space to be vented unless the air in it is "conditioned" thus the terms "closed" and "sealed" should be accompanied by the term "conditioned" to avoid misunderstandings.

Depends on where you are, those terms are used largely interchangably.
Most codes I've seen use 'closed' and 'sealed' as the same (no longer vented) but using a dehumidifier to control the humidity in the space. A 'conditioned' crawl would refer to one using hvac air to control the humidity, with the exterior walls insulated.

I'm not aware of any code that accepts a dehumidifier in a sealed or closed crawlspace.

The IRC applies to almost all of the US and this is what it says:

2006 & 2009 IRC 2006 - Section 408.3 - UNVENTED CRAWLSPACES

Ventilation openings in under-floor spaces specified in Sections R408.1 and R408.2 shall not be required where:

- Continuously operated mechanical exhaust ventilation at a rate equal to 1.0 cfm for each 50 ftò of crawlspace floor area, including an air pathway to the common area, and perimeter walls insulated in accordance with Section N1102.2.8.

- Conditioned air supply sized to deliver at a rate equal to 1.0 cfm for each 50 ftò of under-floor area including a return air pathway to the common area, and perimeter walls insulated in accordance with Section N1102.2.8.

2006 IECC Section 402.2.8 and 2009 IECC Section 402.2.9 - CRAWLSPACE WALLS

- As an alternative to insulating floors over crawlspaces, crawlspace walls shall be permitted to be insulated when the crawlspace is not vented to the outside [excerpt].

"Common Area" is the living space of the house.

"Conditioned Space" is the space within a building that is provided with heating and/or cooling equipment or systems capable of maintaining, through design or heat loss/gain, 50ðF during the heating season and 85ðF during the cooling season, or communicates directly with a conditioned space. For mechanical purposes, an area, room or space being heated or cooled by any equipment or appliance.