Green Screen terminals (and emulators) allowed dynamic control over visual attributes on the screen.
In T27 terminals for example, this was achieved by sending T27 control codes as part of the data for a screen. These codes could be changed within the LDL, thus allowing the programmer to dynamically alter the presentation of certain fields or lines at runtime.
This has often been used to highlight fields or lines to bring them to the user's attention. However, these control codes are device and platform dependent. The same codes did not work for all terminal types in the same way (i.e., T27's would only understand T27 control codes). This prevented migration from one platform to another.
Component Enabler was originally not designed to support such field highlighting capabilities, due to the cross-platform issues. However, since it is still a desired feature by many users of the product, a platform independent interface using dynamic lists has been introduced to convey attribute information for fields on a Component Enabler Client form.