The blend of a microfiber towel is the ratio of the components that make up the fibers of the fabric. Most microfiber is a blend of polyester and polyamide. Some (lower quality) microfiber is 100% polyester (there is some high grade 100% polyester microfiber, which does perform wel, in certain applications). The first number represents the ratio of polyester and the second number the ratio of polyamide.
In general, polyester is the scrubbing and cleaning component, and the polyamide is the absorbent component of the fiber. Polyester is the dominant material in all microfiber products. Polyamide is more expensive than polyester, so all else being equal, fabrics with a more of it, are more expensive.
With high quality fabrics made on the same weaving or knitting machine, a 70/30 material will be softer and more absorbent than an 80/20 material. With some soft automotive paint, detailing professionals, using high grade lighting, can see the micro scratches left by some 80/20 towels, where the same towel in 70/30 leaves no marks behind.
With all of that said, most microfiber material is marked incorrectly (either on purpose, by supplier, without their knowledge, or because of factory cost cutting). The only reason that the blend is marked on the towels (usually with a sewn in tag) is that US Customs requires that all textiles imported be marked with their components/ingredients, and their country of origin. There is no one, or authority out there checking to make sure that fabrics are marked correctly.
It is up to the seller to check this on their own. This expensive lab test can be cost prohibitive. Factories will cut corners to meet price points, and in order to do so will use less expensive materials but leave the marked specifications unchanged. We have done independent tests on one big box stores popular microfiber towels and they were 100% polyester (or 95/5), when marked 80/20. We also have competitors who we know buy from our same factory in South Korea but mark their towels as 70/30 when they are really 80/20.
So what should you do as a consumer trying to select the right microfiber towels for your business or hobby? Only buy from reputable sources, who have consistent production, and quality control. Make sure you evaluate new towels, to ensure that they meet your standards. The advertised (and marked) blend of the towel should not influence your evaluation of the products. Always use your own best judgement!