The lower grades of magnet consist mainly of the basic alloy NdFeB and have a poorer temperature resistance and poorer corrosion properties. If you want to increase the magnetic grade, this is often achieved by adding different alloying elements such as dysprosium and praseodymium. These alloying elements can be used both to increase the temperature resistance and to improve the corrosion properties.
Most magnets are made in China nowadays and have designations which are roughly similar to the ones below.
Designation Temperature and corrosion properties
N 35, 40… 52 approx. 80°C and requiring surface treatment
35, 40… 52M approx. 100°C and requiring surface treatment
35, 40… 52H approx.120°C and requiring surface treatment
35, 40… 52SH approx. 150°C and requiring surface treatment
40 UH approx 180°C and requiring surface treatment
30, 33, 35, 38EH approx. 200°C and requiring surface treatment.