Joined: 07 Nov 2005 Posts: 1105 Location: The Netherlands at Hole 19
A little bit of reading the manual is very helpfull but I'll answer briefly.
Key Feedback Area contains the concatenated keys of the segments in a logical database record; sequence is from top to bottom.
MPP is message processing program and processes messages sent from a terminal or another message handling program.
SPA stands for scratch pad area and is used to store information like switches, numbers and counters you would normally keep in your working storage. Since the application might be (re)loaded between two transactions (communication with a terminal) all fields might be initial. So the SPA helps your application to decide what step it should take after gaining control.