Beware that should youuse the tool
Create Feature Line from Object
and use the XREF option, and attempt to assign elevations to the feature line the tool fails!
It remains broken since it's initial release, and autodesk has yet to admit the issue exists.
In normal operation should one convert a polyline to feature line, and chose assign elevations option, one is then taken to a page to assign a fixed elevation, or allow the feature line to get elevations from the surface the user selects, and all goes well.
With the [XREF] option; when one selects assign elevations and clicks the OK button, one is then dumped out of the command straight to a command prompt. Yes the feature line is created. NO, you did not assign elevations to it, this must then be done by editing the feature line you just made because the command fails. Two years running, let's just see how long it takes them to first admit it doesn't work, and then actually fix the problem.