"Not in RIC" is a supplement to the volumes VI and VII of RIC (The Roman Imperial Coimage). The general arrangement, nearly all conventions and most of the descriptions are taken from RIC. There is also no revolution in coin numbers. For example, "HERACLEA 67" refers strictly to the RIC number and "HERACLEA [after 67]" means that in the author's opinion an unlisted coin should be placed there, i.e. after HERACLEA 67, according to the rules established in RIC. However, it is sometimes an arbitrary decision.
When attributing a particular specimen as unlisted, be sure that you have checked Corrigenda first. Sometimes coin is in fact listed in RIC, but with incorrect or unclear description. For example, very common RIC VII HERACLEA 48 is erroneously described as having bust turned left.
Keep in mind that not all kinds of "unlisted details" are taken into account. For example, unlisted decorations of shields, helmets, altars and so on, are usually omitted. The main reason is that RIC in principle gives only examples of such details, without any intention of being exhaustive. Also legend break is not an essential part of description (cf. footnotes on p. 111-115 in RIC VII with constantly repeated phrases "breaks unknown", "breaks not recorded" etc.). There are, of course, exceptions. Sometimes difference in legend break or in decoration of altar makes a separate entry in RIC tables.
Also, not every single dot/pellet is assumed to be important. Most unexpected "unlisted dots" are accidental (deposits, die cracks, centering marks), as well as missing ones. Again, there are exceptions to this.
Another category are [irregular coins], i.e. ancient counterfeits; so-called "barbarous imitations", "ancient imitations", "imitative coins" or "unofficial coins". Formally, these coins should not be listed among the imperial coinage, but there are quite a lot of exceptions. First, because RIC (especially the volume VII) also mentions them in footnotes. Second, because they are sometimes interesting and could tell us more about a regular coinage. Third, because not infrequently it is hard (or even impossible) to distinguish between official and unofficial coin.
Additionally, there are groups of coins, which are completely absent in RIC VI and VII. For example, anonymous silver coinage (catalogued by Bendall), pagan coinage of the Great Persecution and Festival of Isis coinage (catalogued in Vagi). They are also absent in this supplement, but with very few exceptions.
Remember that there is a section called ABBREVIATIONS. Useful if you need to decipher something like "cuir." or Bikić-Do Hoard.
There is also a simply site search engine accessible from the main page. It could be useful as well.