Topline to me means more muscle over the length of the back and up the neck. I may be corrected on this but heres how i see it.
1st photo of Inky straight from the field, clipped and with no topline... and 2nd after conditioning feeding program and exercise for getting ready for the show ring having gained topline..
To me you can see how the after pics show he has more muscle definition over his bottom and back.