3270 allows user to edit screen contents without interaction with host computer and allows host computer to mark screen regions as user editable or not. Interac

3270 allows user to edit screen contents without interaction with host computer and allows host computer to mark screen regions as user editable or not. Interaction with host mostly consists of sending (text) framebuffer contents back and forth. SAP's dynpro is abstraction layer on top of that which presents somewhat higher level view of that (forms and fields, not framebuffers and characters) and also allows 3270's behavior to be emulated on normal character-oriented terminals (obviously by ncurses-like software running on host). This abstraction is sufficiently high-level that sapGUI can look like somewhat modern desktop application instead of obvious 3270 emulator. (Windows version of sapGUI allows you to render parts of UI as embedded HTML or ActiveX components, but that capability does not seem to be used that much outside of SAP's development tools) From programmer's point view it is not that different from web 1.0 application built on semi-modern form handling library (eg. WTForms or whatever your all-in-one webapp framework provides). Probably only difference is that for web frameworks you will have if(form_submitted_and_valid()) somewhere, while in SAP body of that conditional and rest of the controller implementation are always two distinct procedures.