When a drip tip is said to be a 510, it means it is meant to fit atomizers and cartomizers which use the 510 connector, assuming it has a replaceable drip tip. There are other sizes, though they've become less and less popular over time and are getting rare. Also, there are often slight variations in 510 drip tips. For instance, I bought a really tall drip tip to use on my Nimbus, but it won't fit. It's too big, even after using several different o-rings. However, that same tip fits perfectly on my Protank2 and Protank2 mini, as well as my Boge 510 cartos. Oddly, both of the Protank2's driptips fit the Nimbus just fine. So they're not *all* interchangeable, as one might think. So if you are looking to buy a drip tip for a particular atty, check the reviews. Often people will say 'I bought this for such and such atty but it doesn't fit', or 'it fits perfectly on my _'. It's not a perfect guide, but sometimes it can help.