This option is only available when you import two or more datasets to ENVI Lidar. According to the ENVI Lidar help: Source ID is a value marked for each file when files are imported into a project. When multiple input files are imported, a different source ID is used for each file. If only one input file is imported, the source ID is not used. To show point features by source ID, the project must contain two or more input files that contain multiple, distinct source ID values.