Stain and Interior Stain

The answer to your question is buried somewhere in SCAQMD staff technical reports that accompanied the rule making. Sometimes the regulatory approach may seem counter intuitive. Each category and subcategory limit in the Table of Standards is based on extensive technical review of technology that might be available at the time a category limit goes live. This is the SCAQMD take on Best Available Retrofit Control Technology (BARCT). The staff reports are usually several hundred pages long. Notwithstanding later category limits, the limits in effect on January 1, 2004 are the basis for LEED credit conformance.

Without digging into the staff reports, my best guess is that the exterior stain limits were lower than interior based on intended use and formulation technology. While there are niche stains for materials like concrete and masonry, the majority of stains are designed for wood products. Many of the exterior stains are latex or polymer based and form a film. Many of the interior wood stains are designed to impregnate the pore structure of the wood with fine pigments. These are sometimes single step products and sometimes require a clear topcoat. The wood is more likely to be a hardwood species with a tighter pore structure than something a deck or siding might be made of. The resin technology is not necessarily the same as with the exterior products.

The technical nuances lead to confusion. It is always best to look at the Table of Standards in context with the definitions at the beginning of Rule 1113. People like me on the regulatory compliance side start there and sometimes dig into staff reports and other technical documents. Ideally, the manufacturer you are dealing with should have already done the categorization work for credit conformance so you don't have to.