Neodymium magnets are graded according to their magnetic strength. The higher the grade (number) the stronger the magnet. The letter or two letters that may follow the number indicate the Intrinsic Coercivity (Hci) of the material.
The higher the Hci, the higher the temperature the material can be exposed to before the magnet will start to show permanent losses in output. This is what links the letter or letters to a temperature rating.
More Reading: Neodymium Magnet Material Grades
The current range of Neodymium Iron Boron magnets is as follows:
N27, N30, N33, N35, N38, N40, N42, N45, N48, N50, N52, N30M, N33M, N35M, N38M, N40M, N42M, N45M, N48M, N50M, N30H, N33H, N35H, N38H, N40H, N42H, N45H, N48H, N30SH, N33SH, N35SH, N38SH, N40SH, N42SH, N45SH, N28UH, N30UH, N33UH, N35UH, N38UH, N40UH, N28EH, N30EH, N33EH, N35EH, N38EH, N33VH/AH.
Rare Earth Magnets create hazards not seen with other types of magnets. When handled improperly, they can pose serious dangers. Even small Neodymium magnets are strong enough to cause injuries to body parts pinched between two magnets.
Magnets allowed to strike each other with enough force will chip and shatter the brittle material.