Natural determines when to open Work (sequential) files through the Natural Parameter (NatParm) WORK (or the macro NTWORK). WORK specifies whether the files will be opened upon execution of a module containing a READ WORK or WRITE WORK statement, or whether the open is deferred until the specific statement is executed. The choice is significant if execution of the statements is conditional.
Natural will automatically close all open files at the end of a session. You can close a file explicitly with the CLOSE WORK statement. After the CLOSE WORK, Natural will re-open the file upon execution of another READ WORK or WRITE WORK.
The files remain open across subprogram calls (CALLNAT), so in your example, there is only one open and one close per Work file.
My preference is to open files upon statement execution.