Before executing any commands, or shifting screens, the current screen is read. If there are any input controls, these are checked and their contents recorded. If the contents are not valid, a warning message reporting this is presented, the offending input is highlighted, and any commands or screen shifts are ignored. If the screen has no input controls, then reading the screen has no effect.
If the user invokes a TEXTMENU or BUTTON control in which the first command is CANCEL, then the current screen is not read before exiting. Note that you cannot use & to combine commands with a CANCEL command — only the CANCEL command will be executed, and the appropriate new screen shifted to.
