In politics and sociology, "Belt" is used after a word to describe a region that has similar characteristics. It originated to describe agricultural regions that grew a certain crop "corn belt", "cotton belt", and was then later used to describe other economic similarities "rust belt", and later similarities of lifestyle or climate "sun belt". Now, much as with the -gate suffix, it is used rather liberally. It is also used to describe demographic areas that are not actually connected, I have recently heard, for example, the term "diploma belt" to describe scattered communities with high education rates.
As with any division of the United States by region, the "belts" are a good guide, but should not be taken too seriously, since they often take one characteristic as being more distinctive and important than it is. For example, within the "rust belt", there are many areas that are depressed manufacturing centers, but there are also agricultural areas within that same region, as well as urbanized areas with prosperous economies.