public List<PointCloudReturnType> ReturnValues {get; set;}

//Must be called on the MCT
//var pcsl = ...;
//Retrieve the available classification codes
//At 2.x - var dict = pcsl.QueryAvailableClassCodesAndLabels();
var dict = pcsl.GetAvailableClassCodesAndLabels();

//Filter out low noise and unclassified (7 and 1 respectively)
//consult https://pro.arcgis.com/en/pro-app/help/data/las-dataset/storing-lidar-data.htm
var filterDef = new PointCloudFilterDefinition()
{
  ClassCodes = dict.Keys.Where(c => c != 7 && c != 1).ToList(),
  ReturnValues = new List<PointCloudReturnType> { 
                         PointCloudReturnType.FirstOfMany }
};
//apply the filter
pcsl.SetFilters(filterDef.ToCIM());

//Must be called on the MCT
//var pcsl = ...;
IReadOnlyList<CIMPointCloudFilter> updatedFilter = pcsl.GetFilters();
foreach (var filter in updatedFilter)
{
  //There is either 0 or 1 of each
  if (filter is CIMPointCloudReturnFilter returnFilter)
  {
    PointCloudFilterDefinition pcfl = PointCloudFilterDefinition.FromCIM(updatedFilter);
    List<PointCloudReturnType> updatedReturnValues = pcfl.ReturnValues;

  }
  if (filter is CIMPointCloudValueFilter classCodesFilter)
  {
    // do something
  }

  if (filter is CIMPointCloudBitFieldFilter classFlagsFilter)
  {
    // do something
  }
}