public static AssetDatabase.RefreshImportMode ActiveRefreshImportMode { get; set; }

This mode determines whether the Editor imports assets in the Editor process, or in parallel in import worker processes, during an Asset Database refresh. For the available modes, refer to AssetDatabase.RefreshImportMode.

The initial value comes from the Parallel Import setting, which you can also read and write with EditorSettings.refreshImportMode. The command line argument overrides that setting.

Unity applies this property to the current session only and doesn't save the value. To change the mode for the project, set EditorSettings.refreshImportMode instead.