Bench type autoclaves have three classifications N, B and S type.
N Type: Older style of sterilisation is achieved by boiling water in the chamber between 121°C/134°C and is good for sterilisation of non-wrapped solid instruments.
B Type: Current best practice of sterilisation is achieved by boiling water or steam injecting into the chamber between 121°C/134°C while also using a vacuum pump to remove the contaminated air periodically during the cycle. This style of sterilisation is good for wrapped and non-wrapped instruments as well as hollow and porous instruments.
S Type: Using a mix of N and B type functionality the S type autoclaves are manufacture specific as some will use a vacuum pump while others may not. This style of sterilisation is good for wrapped, non-wrapped and porous instruments.