By default, these key points have no slope constraints or distance constraints. I am trying to gather experience in how to effectively use them manually. Any additional observations on this would be welcome indeed. I'm sharing mine here.
Distance Constraint
This locks the longitudinal positioning of the key point according to another specified key point.
Bug: The value of Offset seems to be taken as meters instead of working units, in Geopak SS4 .872.
Cross Slope
This locks the slope to the same slope at the already-established key point provided.
Mirror Cross Slope
This locks the slope to the negative of the slope at the already-established key point provided.
Vector Slope
When using the slope constraint Vector Slope with Slope Transition 1 set to the first key point and Slope Transition 2 set to the second key point, what I'm seeing is that it does nothing but allow you to see what the cross slope should be at any given point along the superelevation element. I mean, wherever I place my intermediate key point, the cross slope seems to be simply an interpolation between the two key points. Not too useful, that I can tell. It's conceivable that it might be useful when working with more than one superelevation lane.
Wish list
I'd like to see the option to lock in a rate of change (ROC) coming from a back key point and another rate of change coming from an ahead key point. For example, an ROC of 0% per foot from one and an ROC of 0.06% per foot from another. With those inputs, it would establish an appropriate longitudinal position for the keypoint.