What does the class 6H represent in a hole?

When using Autodesk Inventor to create a hole in a surface to fit an M4 screw in, I come across 4 parameters: size, class, designation, and diameter.

I chose the size of 4 mm, but I don’t understand what class represents, and it seems that the only option for this is for it to be 6H, even though other sources online state that there should be other option such as 6G and 5g6g.

• What do these represent?
• What is meant by class?