There is some confusion out there on what the slope lines mean and how to read them. Here are a few examples to clear things up.
It may look a little messy at first but I actually like these old style slope lines much better than modern day contour lines. It is easier for me to visualize up and down.
In this example, Red is up on top the hill. They can see down to Blue at the lowest elevation, sitting behind the stream. Effect: Red will have cover here if attacked by Blue.
Red has dropped back here behind the ridge now. They are on the Reverse Slope. There is no line of sight between these units. Effect: Red will have cover if attacked by Blue.
In this example, Blue is at the highest elevation. They are up on top of the hill. Red is still on the hill but further down the slope. White is down at the base elevation at the lowest point. Effect: Blue will have cover here if attacked by Red. Red will have cover if attacked by White.
Here, Red and Blue are both on the hill. The issue is that they are both on the same slope. There is no cover here for either side. I would treat combat between these two units, the same as if they were on clear terrain.