NO_SESSION (The default.) Specifies that the opened form should share the same database session as the current form. POST and COMMIT operations in any form will cause posting, validation, and commit processing to occur for all forms running in the same session.
It lists all the values that can be used and what they do, there's only 2 of them.
Check the Forms help on the OPEN_FORM built-in. The Session_Mode parameter has two valid values: NO_SESSION (the default) and SESSION. As 994122 pointed out, the NO_SESSION parameter instructs Forms to share the same session between all forms opened using this parameter. Therefore, any POST or COMMIT will commit data accross all open forms. If you use the SESSION parameter value instead of NO_SESSION, this will cause the form opened to use a seperate session. Any POST or COMMIT issued in this Form session will not affect the calling form.