What does "Grade N42" mean ?

The grade number of a Neodymium Supermagnet indicates how
powerfully the individual magnet is magnetized.

The units of measure of a magnet's strength are named after
Carl Friedrich Gauss, and Hans Christian Ørsted. They were
Scientists who studied magnets in the 1700's.

Ørsted's name is re-spelled Oersted because English does not
commonly use the Danish "Ø", so to make the same sound,
English uses "Oe".

The theoretical maximum for Neodymium is Grade N64
meaning 64,000,000 Gauss-Oersteds.
"Mega" is the scientific prefix for million.
So, Grade N42 would be 42 MegaGauss-Oersteds.
This is often abbreviated as "42MGOe" .
Neodymium Supermagnets cannot obtain this full strength
at normal temperatures, but can approach it at near absolute
zero temperature.

As the alloying mixtures and manufacturing processes improve,
Neodymium Supermagnets will become available in higher grades.

I have read that grade N58 has been reached in the laboratory,
and Grade N45 is available at some magnet suppliers.
Some Neodymium magnets may be graded as low as N28.
Grades above N45 are very expensive and are not commonly available.

These higher grades are more sensitive to de-magnetizing
forces and temperatures, and are often kept super-cold from the
moment of magnetizing until the customer receives it.
Some are even shipped non-magnetized, and the customer
magnetizes them in their own facility.

Neodymium magnets are always "charged to the maximum" and
cannot be "pumped up" higher, but if they were subjected to
reverse fields, mechanical shocks, or high temperatures,
they can be recharged if the right equipment is available.

The buyer should pay attention to the Grade Rating of any magnet
they want to buy. Low cost magnets may not have the same "pull"
as more expensive magnets of the same size.