#1
"if the table of contents is also acting as the definition for the ECO codes" ++ Yes
"the TOC is describing what's in those sections" ++ Yes
"is it also describing the ECO code itself" ++ Yes
"those TOC entries could be written out as a kind of key or taxonomy?" ++ Yes
"it seems as if there's a definition for each layer" ++ No. The layers beyond the 500 ECO codes A00 to E99 are just numbers and they change with each edition of ECO
"Italian Game: Hungarian Defense", "Giuoco Piano Game: Giuoco Pianissimo, Italian Four Knights" "it would seem there are seven named layers underneath its own two named layers of definition" ++ That is the beauty of the ECO system: it is shorter than the names, it is unambiguous, it does not depend on language or local naming customs. The ECO is a partition of all openings into 500 classes A00 to E99.
"we can attach a description to C 5 and that's the definition for ECO C 50 - C 59?" ++ Yes
"we can attach a description to C 50 and that's the definition for ECO C 50/001 - C 50/007?"
++ Yes and no: C50 is C50/001 to C50/999, but what C50/007 is can change. If C50/003 is heavily played then in a next edition it may be split into new C50/003, C50/004, C50/005. If C50/006 and C50/007 do not get played often, then in a next edition they might be merged into a single C50/006.
Wondering if the table of contents is also acting as the definition for the ECO codes, and if so, how to write that out, as more than just a table of contents. For instance, this is how C 5 is defined:
1.e4 e5 2.Nf3 Nc6 3.Bc4
and if you go over to the C 5 section (that's C50-C59) you get this for C 50
1.e4 e5 2.Nf3 Nc6 3.Bc4
-- various moves without 3...Bc5 or 3...Nf6
-- 3...Bc5 followed by various moves without 4.b4 or 4.c3
and if you go over to the C 50 section, you get two lines (variations?) that follow from
1.e4 e5 2.Nf3 Nc6 3.Bc4
and then you get five lines (variations?) that follow from
1.e4 e5 2.Nf3 Nc6 3.Bc4 Bc5 4.d3 Nf6 5.Nc3 d6
So here's where I start to wonder. First off, clearly the TOC is describing what's in those sections. But is it also describing the ECO code itself, as in those TOC entries could be written out as a kind of key or taxonomy? It would certainly seem to be the case, but perhaps I'm missing the point? Maybe there is no actual definition for each code, because each code is merely describing a collection of variations? But variations on what?
I can see, for instance, that C 5 (C 50 - C 59) provides lines for the Italian Game. It only specifies that one thing. Not even what it isn't. C 6 is something to do with the Ruy Lopez, and C 4 is for similar lines that don't lead to either of those two.
Then I can see that C 50 is for *some* lines related to the Italian Game.
Each of those codes described in the TOC would need the TOC description to act as a definition, because of the extra conditional stuff. So if they are actually definitions (???) then it seems as if there's a definition for each layer. Because, for instance C 50/001 -- if I run the line through analysis -- comes up Italian Game: Hungarian Defense. If I run C 50/002 through the analysis page, it turns up Giuoco Piano Game: Giuoco Pianissimo, Italian Four Knights Variation. So for C 50 it would seem there are seven named layers underneath its own two named layers of definition (those two lines that I wrote out above, which work out, in the analysis page, to Italian Game and Giuoco Piano Game: Giuoco Pianissimo, Italian Four Knights Variation.) And for C 5 it would seem that the named layer of definition is Italian Game.
So, in other words, since I'm trying to write out the taxonomy on my website, to hopefully provide context, I just need to establish that context clearly. These are levels of hierarchy, right? And each one has a clear description of what's contained within? And we can attach that description to that particular code? So we can attach a description to C 5 and that's the definition for ECO C 50 - C 59? And we can attach a description to C 50 and that's the definition for ECO C 50/001 - C 50/007?
That's where I'm at.