What do they mean

The number was assigned to the driver. The cars don't have numbers, drivers have numbers. If your friend drives the car the number on the window must be changed.

To race in a traditional SS class (SS/J) it needs to have an engine that was available new in the car. You could race it in SS with a 340 and then it would be in something like GT/H for example. The non original engine you choose must be approved by NHRA for SSGT and many of the popular choices are approved. You would put an A at the end for an automatic. GT/HA

You could put a challenger drag pack engine in the car and then it would be in a SS class like FGT/F. Which means factory GT F. These don't have an A for automatic so you don't know the trans by the class on the window. They all run together.