⎕SE is a system namespace.  Its GUI components (MenuBar, ToolBar, and so forth) define the appearance and behaviour of the APL Session window and may be customised to suit individual requirements.

⎕SE is maintained separately from the active workspace and is not affected by )LOAD or )CLEAR.  It is therefore useful for containing utility functions.  The contents of ⎕SE may be saved in and loaded from a .DSE file.

See Dyalog for Microsoft Windows UI Guide for further details.