Because many official IIDX song conversions were removed from major BMS servers due to copyright concerns, players often rely on community-hosted mirrors to find legacy packs (e.g., songs from Gameplay Modification:

The Evolution of IIDX BMS: Understanding the Mirror Community In the niche world of rhythm gaming, the intersection of beatmania IIDX (Konami's flagship arcade title) and Be-Music Source

resources/README.md at master · BMS-Community/ ... - GitHub

In this context, the term "Mirror" usually refers to one of two things: File Repositories (Mirrors):

For many, BMS is not just a game but a tool for breaking past the skill ceilings of the arcade version. While IIDX arcade difficulty caps at Level 12, BMS difficulty tables (such as the "Insane" or "Overjoy" tables) push far beyond those limits.