The MS_LEVEL_LOAD_ATTACHMENT_FILTERS configuration variable, if set, loads level filters of reference attachments when the master file is opened. For every unique attachment by file, a top-level filter is created. This top-level filter represents the reference attachment. All filters of the reference attachment are loaded as children of this new top-level filter. The reference filters may be modified in the context of the current session. These changes are not persisted in the DGN file and are discarded when the session is terminated.