---Mod (in-game)---HR: Hard Rock
HD: Hidden
SD / PF: Sudden Death / Perfect
FL: Flashlight
EZ: Easy
NF: No Fail
HT / DT: Half Time / Double Time
---osu!mania Difficulty---EZ: Easy
NM: Normal
HD: Hard (Don't confuse this with mod!)
MX: Maximum
SC: Special
---Mod (beatmap)---SV: Slider Velocity
LN: Long Note. Use with osu!mania notes.
RQ: Request
NM: Normal Modding (Don't confuse this with osu!mania Difficulty!)
M4M: Mod 4 Mod
GD: Guest Difficulty
---Mode---STD: Standard
CtB: Catch the Beat
o!m: osu!mania
I'm quite sure there are more than this but I don't know.