The manuals
EDT V17.0A Unicode Mode Statements
EDT V17.0A Unicode Mode Subroutine Interface
describe EDT's Unicode mode. The manuals
EDT V16.6B Statements
EDT V16.6A Subroutine Interface
contain a description of the compatibility mode.
In addition, the EDT V17.0A Statements manual contains a section describing the extensions in the compatibility mode compared to EDT V16.6B.
The manuals dealing with the EDT statements describe the fundamental concepts of EDT in each of these modes and can be used as a reference for the many EDT statements.
The manuals dealing with the subroutine interface describe how the user can write programs which can be called by EDT or which themselves call EDT as a subroutine can be programmed. These manuals can only be used properly in combination with the manuals describing the EDT statements.