ECO ( or Encyclopaedia of Chess Openings) is a classification of chess openings moves.The moves were taken from hundreds of thousands of games between masters, from published analysis in the Chess informant since 1966.
Instead of the traditional names for the openings, ECO has developed a
unique coding system that has also been adopted by other chess
publications. There are five main categories, "A" to "E", each of which
is divided into one hundred sub-categories.
ECO code is a trademark of Chess Informant Inc.
Category A contains the openings which are considered as flank openings( i.e the attack using the minor pieces rather than the pawns such as in the e4 or d4 openings).
they also include the unusual openings which are denoted by A00.