I think this is the meaning.

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Note added at 7 hrs (2015-04-16 23:50:10 GMT)
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Calculating VOC Content of a Material

The regulatory VOC calculates the VOC less exempts and water, which is a more complicated calculation than the actual VOC, in that it subtracts the volume of water and the volume of exempt compounds from the volume of material, in the denominator. he calculation was derived to express the VOC emitted per volume of coating solids to eliminate the effect of dilution. Dilution with water or exempt solvents would reduce the VOC-to-paint-volume ratio while maintaining a constant VOC-to-paint/adhesive-solids ratio. This is important because those materials are applied at a certain film thickness so dilution would result in a larger volume of the material being applied to achieve the same film thickness. For conventional solvent based products with no exempt compounds, the two values are always the same.
http://www.aqmd.gov/home/regulations/compliance/vocs/calcula...

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Note added at 10 hrs (2015-04-17 02:58:56 GMT)
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VOC-Exempt Solvents EPA regulations include a list of compounds that are explicitly exempted from regulation as VOCs, even though they are “compounds of
carbon.”
This list of compounds (often referred to as “VOC-exempt compounds”)
is established and modified by regulation.
http://msdssearch.dow.com/PublishedLiteratureDOWCOM/dh_0033/...
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This memorandum is to clarify that exempt solvents should be subtracted out from
coatings just like water, with the ultimate value of interest being the mass of VOC per unit volume of coating less exempt solvent and water.
http://www.epa.gov/region7/air/nsr/nsrmemos/excluson.pdf
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Definition - What does Exempt Solvent mean?
An exempt solvent is a particular type of solvent that falls below specific safety criteria. In certain instances, there are solvents that do not entirely comply with the safety use standards, but are still permitted for use. When a solvent is said to be "exempt," it qualifies or is exempted.

Other terms to describe it include volatile organic compound (VOC) exempt. This simply means the solvent is safer than similar options.

Corrosionpedia explains Exempt Solvent

The use of solvents in various industries has been regulated by state regulators for a very long time. The early classification of exempt solvents was mainly applied to methyl chloroform, methyl chloride and other types of halogenated products. Ideally, volatile organic compounds should be excluded from certain solvents, but there are circumstances where its addition is inevitable; so as the identification of exempt solvents.

For instance, methyl chloroform is a potent ozone destroyer, making it unsuitable for products like coatings and paints. Such materials also evaporate very quickly, which may not be good when used in coatings. The rapid rate of evaporation of halogenated products combined with low boiling points could lead to leveling problems and severe flow problems.

Industries must be critical and adhere to the existing policies when on the use of solvents. This is to guarantee that products like coatings reach their maximum potential for corrosion protection and other properties.
http://www.corrosionpedia.com/definition/478/exempt-solvent