Limitation: it does work for linears created by a template (corridor or linear template). For other 3D linear features, you must use Last Mile Utilities tool.

Principle:

• set linear 3D annotation group that will be assigned to top feature definition
  The annotation group is defining target features (possible bottom of slope from top feature) and style
• assign this annotation group to top feature defintions

youtu.be/jBtmlyjpmoA

Example:

• Simple case where it is easy to identify top and bottom
  Linear 3D annotation group must be assigned to End Cond Cut Tie. It looks for End Cond Dit out
  
• "Complex" case
  
  
  Cut and Fill are creating single Construction Limit_L feature
  You don't know the feature definition that will be used. It depends on  the first one created, cut or fill
  Here, Fill 3D Linear annotation group must be assigned to Rail_Subballast feature definition seeking End Cond Fill Tie  and End Cond Cut Tie as bottom features.
  
  Cut 3D linear annotation group must be assigned to both End Cond Fill Tie  and End Cond Cut Tie seeking End Cond Dit Out as bottom features.