To grade pearls correctly, a standard System covering A AAA is used worldwide. This means, the highest quality of pearls is labelled with AAA. Sometimes, merchants invent new classifications like AAA+ or AAAA. These invented grades are meant to mislead customers and to delude about the quality.
This system grades pearls (Perlen) on a scale from AAA to A, with AAA being the highest grade, this grading scale is common to freshwater and akoya pearls only, but is accepted by many with South Sea and Tahitian pearls as well:
AAA: The highest-quality pearl, virtually flawless. The surface will have a very high lustre and at least 98% of the surface will be free from any type of defect.
AA+: The surface will have a high lustre and at least 90% of the surface will be free from any type of defect.
AA: The surface will have a very high lustre and at least 85% of the surface will be free from any type of defect.
A: This is the lowest jewellery-grade pearl with a lower lustre and/or more than 25% of the surface showing defects. In many cases, if the pearl is being mounted into a piece of jewellery, it can be mounted so that the defects are hidden -- thus providing a lovely jewellery piece at a lesser price.
Some reputable sellers may use intermediate grades for the pearls - only those that do not fall in a category but are between two - such as A+ and AA+. Obviously these grading categories are quite broad and leave room for interpretation and individual judgement. Also note, that in multi-pearl pieces such as strands, necklaces, bracelets, etc., all of the individual pearls may not absolutely meet the indicated grade level. For example, a strand referred to as "AAA" must have most of its pearls as AAA pearls.
However, a few pearls could have slightly lower lustre or a tiny bit more surface defects. This is because matching is also a primary consideration in multi-pearl jewellery, sometimes even overriding a very strict grading of each individual pearl.