ECO classifications

In my CB Light Prem DB I have games that show ECO such as D01/09 or B14/13. I understand the basic class, e.g., D01 but what is the suffix (/09 or/13)??

Thanks.

It's the line: Code D01 line 09

Thank you Rgoltsman and forgive me for being such a dummy......line 9 where? In what context?

Thanks again.

In the Encyclopedia of Chess Openings which is what ECO stands for.

Thanks Sharukin. I have the ECO codes but they are in a truncated form so the line number did not mean anything. I found the full ECO version and sure enough the line number matches.

https://youtu.be/grGgUk6tu_M

Are such line references consistent from one ECO revision to the next?

No. New lines can be added, and I don't think they add them at the bottom.

I guess that you are now neither the one on the left nor the one on the right.

I just work on harmony and diction.

Lines might also disappear. This image shows E46 in 1978 (left) and 1998 (right) print editions.