The top-level class is FDynamicMeshChangeTracker, found at the bottom of this file. You create an instance of this and then call BeginChange(), then call SaveVertex()/SaveTriangle() before you make modifications to a vertex/triangle, and then call EndChange() at the end. This function emits a FDynamicMeshChange instance, and you can call Apply() to apply/revert it. So this is the object you would store in a higher-level undo/redo record, for example.