Sounds like they want you to get rid of any organic material under the crawlspace. This is nesessary due to the decay factor that you encounter with organic stuff such as wood, grass, loam or even bugs. Replacing with red clay is the best for your area.
Here in the north we usually use crushed stone or sand.
Another thing you can do to help your building last longer is to lay a 6 mil. plastic over the clay to create a moisture barrier so the humidity does not build up in the crawl space. Not doing this leads to dryrot or mold conditions.
The plastic is cheap insurance to head of problems later.
Good luck on your project,