Might be a stupid question

What is the difference between an
A body fast back
Or an
A body hard top
I was under the impression that duster /dart sport / demon were fast back
Dart swinger/ scamp etc were hard body or hard top
I’m looking at ordering a gull wing for my 74 duster
And they list 2 order numbers
One for a fast back one for a hard body

am I mistaken or misinformed