I'm pretty sure it means that the coin is or was part of a set registry. I have a couple coins with lables like that. Pretty much worthless info if the coin is sold and no longer part of a registry. For instance, series 14 is for Lincoln cents. And 1918 D is the 27th coin of the series. This one of mine used to be in a registry
It is telling what the coin is, numerically. Each coin is part of a series and each coin that is unique to that series has a sequential number. These were used from 1998 to about 2006 for most coins. They have since dropped them at least for US coins, I believe. It has nothing to do with an individual owner of the coin having it in a set registry or not. For example, the 1918 D cent is always a series 14 and coin 27. The one sequentially before this one, a Philly one would be series 14 coin 26 and the one after, a 1918 S would be series 14 coin 28. The sequence for same years, different mintmarks follows a P-D-S format.
