The below code snippet shows generating the feature definitions names used in DGN file category wise for terrains, alignments, and profiles.

//Required References
using System;
using System.Collections.Generic;
using Bentley.CifNET.SDK;
using Bentley.CifNET.GeometryModel.SDK;
using System.Diagnostics;
using Bentley.MstnPlatformNET;

 public void GenerateUsedFeatureDefinitionNames()
        {
            try
            {
                //Get active DGN model
                Bentley.DgnPlatformNET.DgnModelRef dgnModel = Session.Instance.GetActiveDgnModelRef();
                using (ConsensusConnection con = new ConsensusConnection(dgnModel))
                {
                    //Get geometric models
                    IEnumerable<GeometricModel> activeGMs = con.GetAllGeometricModels();

                    //Feature definitions used for Terrains
                    foreach (GeometricModel gm in activeGMs)
                    {
                        Trace.WriteLine("FeatureDefinitions used in Terrains: ");
                        foreach (TerrainSurface terrain in gm.TerrainSurfaces)
                        {
                            if (terrain != null && terrain.FeatureDefinition != null && terrain.FeatureDefinition.Name.Trim() != "")
                            {
                                string fName = terrain.FeatureDefinition.Name.Substring(terrain.FeatureDefinition.Name.LastIndexOf("\\") + 1);
                                Trace.WriteLine(fName);
                            }
                        }
                    }

                    //Feature definitions used for alignments
                    foreach (GeometricModel gm in activeGMs)
                    {
                        Trace.WriteLine("FeatureDefinitions used in Alignments: ");
                        foreach (Alignment alignment in gm.Alignments)
                        {
                            if (alignment != null && alignment.FeatureDefinition != null && alignment.FeatureDefinition.Name.Trim() != "")
                            {
                                string fName = alignment.FeatureDefinition.Name.Substring(alignment.FeatureDefinition.Name.LastIndexOf("\\") + 1);
                                Trace.WriteLine(fName);
                            }
                        }
                    }

                    //Feature definitions used for profiles
                    foreach (GeometricModel gm in activeGMs)
                    {
                        Trace.WriteLine("FeatureDefinitions used in Profiles: ");
                        foreach (Alignment alignment in gm.Alignments)
                        {
                            IEnumerable<Profile> profiles = alignment.Profiles;

                            foreach (Profile profile in profiles)
                            {
                                if (profile != null && profile.FeatureDefinition != null && profile.FeatureDefinition.Name.Trim() != "")
                                {
                                    string fName = profile.FeatureDefinition.Name.Substring(profile.FeatureDefinition.Name.LastIndexOf("\\") + 1);
                                    Trace.WriteLine(fName);

                                }
                            }
                        }
                    }
                }
            }
            catch (Exception ex)
            {
                Trace.WriteLine(ex.Message);
                return;
            }
        }