i am not sure on slant angle maybe like polorazation angle like with satellittes? Elevation angle is the RF take off angles from a antenna in relation to true horizon like in 5 degrees up from true horizon, Azimuth Angle is in Which direction where most RF is going in like in a beam say you are pointed South That would make your Azimuth Angle 180 Degrees and for a omni directional antenna you are using 360 degrees around with hopefully low takeoff angles
and for propagation into the different ionshphere regions. i like to think like skipping a flat stone across water thinking as water as being the ionazation area Certain angles(and also the criticial angle) would hit the water and Reflect off the water other angles would be to steep and enter the water. so certain Elevation angles and frequency for a given Ionzation Area could be to steep and pass through that area into space , but for lower frequencies steeper angles can be used which thus produces shorter hops, very low angles would produce very long hops but remeber everything is in some relation with something else.
i thought i would add my two cents worth hope it enhanced the other guy explantion sorry for spelling as well