It's like Intel i3/i5/i7/i9, or nvidia 50/60/70/80.

It is meant to give a rating inside a generation. Different generations are not necessarily comparable. an i7 is better than an i5, but not if you compare an i7 7xxx with an i5 14xxx.

A nvidia xx80 card is better than a xx60 card, but not if you compare a 1080 to a 5060.

The only difference for OpenAI is that not every generation has every model.