What is the difference between SET SCREEN and CALL SCREEN ?

With SET SCREEN, the current screen simply specifies the next screen in the chain. control branches to this next screen as soon as the current screen has been processed. Return from next screen to current screen is not automatic. It does not interrupt processing of the current screen. If we want to branch to the next screen without finishing the current one, use LEAVE SCREEN.

With CALL SCREEN, the current (calling) chain is suspended, and a next screen (or screen chain) is called in. The called screen can then return to the suspended chain with the statement LEAVE SCREEN TO SCREEN 0. Sometimes we might want to let an user call a popup screen from the main application screen to let them enter secondary information. After they have completed their entries, the users should be able to close the popup and return directly to the place where they left off in the main screen. Here comes CALL SCREEN into picture. This statement lets us insert such a sequence into the current one.