Shetland wool is described in the breed standard as follows: extra fine, soft texture, longish, wavy, and well closed. This is also a good description of the historical Shetland wool, though the terms silky or cottony come up occasionally to describe it, as well as the word short. In this blog, I've repeatedly described what those terms mean (as a side note, the past several years of blog posts are in archive right now for publishing at a later date in another form).
So, to help everyone, here is an 'in-progress' updated form. It differentiates between the informally recognized 'types' of Shetland wool - Flockbook, SSS, and Dual Coated.
Flockbook type is the breed standard type and ranges from grade 1-3. This type allows for fleece that can be cobweb lace fine to an outer sweater wear garment, the typical range for Shetland fleece.
SSS type is the SAME as Flockbook/breed standard but also includes a premium quality for exceptional softness, fineness, and handle. This is due to lower SD/CV and usually CEM as well. These numbers lower the SF (spin fineness) lower than the AFD, which is the hallmark for premium, exceptional handling Shetland fleece. Many SSS breeders, Flockbook breeders, and Fine Fleece North American breeders have intensely focused on breeding Shetland sheep with these premium handling fleeces. The range is from grades 1-3, the same as the Flockbook type. The designation of this type - SSS - is in no way a denigration of the Flockbook type, but only a clarification between the slightly higher SD and CV of the average Flockbook fleece as found in fleeces 'out of the bin' in the wool warehouse and an exceptionally fine quality Shetland fleece. Just think of the SSS type as the 'bulls eye' of the breed standard.
Dual coated type can incorporate the breed standard, but also falls outside that range to include strong guard hairs (which are not described or mentioned in the breed standard). This type includes grades 4 and 5. (Note: the word 'primitive' is not used to describe this fleece because, for Shetlands, very fine, crimpy Flockbook/SSS type fleeces are ancient, primitive, and well described in historical documents). You can be raising Shetlands in the US with pure first importation stock (Dailley stock) and still be raising Flockbook or even SSS fleece type. Dual coated type just refers to a dual coated fleece that has a higher SD and CV, because of the presence of guard hairs, and sometimes a higher AFD, producing fleece that is longer (>6/7"), less crimpy, less bouncy, and coarser. The wool must be spun bulkier than normal or be hand separated into undercoat and guard hairs. The SF is higher than the AFD, meaning that the fleece spins coarser than the AFD.