What's the difference between these, are they just difficulty level for Championship speedruns? I'm wanting to get into IL's but don't see this option

As written in the category rules, there are vehicles that can be unlocked by progressing through the championship. As for ClassA/B in the IL category, the following vehicles may or may not be allowed.

• RX-7
• GT-R
• NSX

I see, thank you very much