Building an MDI frame window – PB Docs 2018

When you create a new window in PowerBuilder, its default window
type is Main. Select mdi! or mdihelp! on the General property page to
change the window to an MDI frame window.

Using menus

When you change the window type to MDI, you must associate a menu
with the frame. Menus usually provide a way to open sheets in the frame
and to close the frame if the user has closed all the sheets.