The Nail Fetish
Not the Sexual Slang
Someone who has a Sexual Nail Fetish isn't automatically a Hand or Foot Fetishist, nor vice versa. Having a sexual affinity towards Fingernails, Toenails doesn't mean that one's hands or feet are even of any sexual appeal to the individual.
There are those whom focus their sexuality on one's nails being dirty, such as grit being under the nails; and those whom focus upon the shine of polished nails. Here, however, we are taking a look when one's Fetish for Fingernails being focused on Action, not their Visual Appeal.
Many Fetishes include, or have sub-Fetishes, which are centered around what the object does or can do; rather than the focus being on the item itself. Just as high heels may be enjoyed for their BDSM/CBT uses in the Fetish World, nails can be enjoyed for their abilities to scratch & scrape.
Not only are such acts of the nails popular in BDSM communities; but they, as a sexual focal-point, have been taking place for quite some time.
In fact, although most people erroneously see the Kama Sutra as ancient pornography, nails being an important part of sexual play (and not just as mere foreplay) has an entire chapter dedicated to it inside its explicitly explained pages.
(I must quickly give note that it is the "Kama Sutra", not the 'Karma Sutra' as many erroneously call it. If you are one of those whom state it incorrectly, it is best to spell/say Karma in its alternate English spelling of "Dharma" as a means to avoid confusion.)
Those with nail-based Fetishes, but probably not the average Jack or Jill, would agree with the Kama Sutra giving the act of digging into one's skin with your nails the same level of importance as Kissing, Caressing, & Moaning.
The Kama Sutra's chapter on "Pressing & Marks of the Nails" speaks of numerous aspects of Nails (like some Psychosexual Nail-Fetishist's Handbook). It speaks of how a woman's nails should look depending on her sexual experience, Such looks, or rather styles, range from the common crescent-shape to having "two or three ridges, as if the blade of a saw." It goes into great detail about what sort of marks (via digging, pressing, and scratching with one's nails) to make upon a lover, and where & when to make them. Indeed does the Kama Sutra place weighted importance upon using one's nails as a means of greatly increasing the pleasure obtained by both partners.