Whatever the code page, the LONGCHAR can only contain one string in one code page. It contains only 1 NULL terminator of its own code page. LONGCHAR cannot consist of multiple code pages, with different character sets in different frames, as an HTML file can.
The UNDEFINED code page is not allowed for LONGCHAR values.
You can use only the large editor for viewing a LONGCHAR (VIEW-AS EDITOR LARGE).
A LONGCHAR program variable cannot be UNDO.
You cannot use a LONGCHAR program variable in WHERE clauses.