No. For all you youngsters that don't understand anything that is not on the Internet, the Encyclopedia of Chess Openings was a 5-book encyclopedia of lines established as being sound by Grandmasters. There were codes - 500 of them.
Each book had a letter, A through E. Each of those had 100 sections (00 to 99). Now keep in mind, this did not account for transpositions. If you transpose, the ECO Code of your game changes.
For Example, 1.e4 b6 is Owen's Defense. ECO code B00 covered all openings where White played 1.e4 and Black DID NOT play 1...e5, 1...e6, 1...c5, 1...c6, 1...d6, 1...g6, 1...Nf6, or 1...d5.
If you play 1.e4 e6, you are in the French Defensr, which is C00 through C19, if White plays 2.d4, and you play 2...b6, you transposed to 1.e4 b6 2.d4 e6 (B00), so the move 2...b6 would not be covered in C00.
The moves 1.e4 e6 2.d4 d5 3.Nc3 Nf6 is covered in C11 to C14.
C11 is 1.e4 e6 2.d4 d5 3.Nc3 Nf6 4.e5
C12 is 1.e4 e6 2.d4 d5 3.Nc3 Nf6 4.Bg5 Bb4
C13 is 1.e4 e6 2.d4 d5 3.Nc3 Nf6 4.Bg5 Be7 5.e5 Nfd7 WITHOUT 6.Bxe7
C14 is 1.e4 e6 2.d4 d5 3.Nc3 Nf6 4.Bg5 Be7 5.e5 Nfd7 6.Bxe7
But 1.e4 e6 2.d4 d5 3.Nc3 Nf6 4.exd5 exd5 would TRANSPOSE to C01, so you would have to know to go there. Normally, that would be reached after 1.e4 e6 2.d4 d5 3.exd5 exd5 4.Nc3 Nf6.
So if you play something outside of this, you are said to be "Out of Book". Like 1.e4 e6 2.d4 d5 3.Nc3 Nf6 4.Bg5 h6, you are out of "book".
ECO E covered Catalan, Bogo-Indian, Queen's Indian, Nimzo-Indian, and Kings Indian.
ECO D covers ALL 1.d4 d5 (with or without 2.c4) and the Grunfeld.
ECO C covers 1.e4 e5 and 1.e4 e6
ECO B covers all other 1.e4 openings.
ECO A covers everything else. Flank Openings and 1.d4 openings not covered in D or E, like the Torre, Trompowsky, Old Indian, Benoni, Benko, and Dutch.
Does the term simply refer to the player in question’s preparation? Or in order for a move to be considered “theory,” does it need to be within the mainline/played by grandmasters/etc.?