: Each puzzle entry includes the starting FEN, the move sequence to solve it, the puzzle's rating, and "Themes" tags (e.g., fork, pin, mate in 2).

These zip files are typically PGN (Portable Game Notation) collections structured so you can practice tactics specifically related to certain openings:

The phrase refers to a popular community-curated dataset where millions of Lichess puzzles are organized into zip files based on their Encyclopedia of Chess Openings (ECO) codes . What the Zip Files Contain

While the official Lichess database provides puzzles in a single large CSV or BZ2 file, the "By ECO" organization was a specific project shared by community members: download all puzzles - Lichess.org

: These are designed for offline training or for use in external chess software (like Scid vs. PC) where users want to drill tactics that arise from their own opening repertoire. Where to Find the Data

: Puzzles are grouped into 5 main folders (A through E) and subdivided into 495 specific PGN files by ECO code.