Looking over the 9440 spec sheet, a question arises. The very first line notes that it has eight registers... eight? There's the four AC's, PC, carry bit (all 16-bits for that one!) and... what? On the NOVA3 you would have SP and SO, but this didn't have a stack. Anyone know?
Try reading as far as Pages 2 and 3 of your data sheet:
Four general purpose registers, program counter, and three internal special-purpose registers.
Calling something which "holds the data being transferred from the ALU to the destination accumulator register for one clock cycle" a "scratch register" might have been done by the marketing department who wanted more registers, not the chip designers, but who knows.