Different NIMH type batteries; what do they mean?

[Small Moose Ltd]

I'm a little green when it comes to this area. I was checking out a website
in the UK for a store that has replacement batteries for Duo's, and they
start referring to NIMH type I through to NIMH type IV.

Can anyone tell me what the difference is between the NIMH's?
Right now it appears that the type I's are much cheaper than the type III or
IV's and I'd hazard a guess that the type III and IV's offer improved
battery life.