De acordo com Wikipedia, CUA é ...
Common User Access (CUA) is a standard for user interfaces to operating systems and computer programs. It was developed by IBM and first published in 1987 as part of their Systems Application Architecture. Used originally in the OS/MVS, VM/CMS, OS/400, OS/2 and Microsoft Windows operating systems, parts of the CUA standard are now implemented in programs for other operating systems, including variants of Unix. It is also used by Java AWT and Swing.
Em outras palavras, um conjunto de padrões / diretrizes para aplicativos interativos a seguir.
Para responder mais especificamente à pergunta, um editor de CUA seria simplesmente um editor de texto que segue os padrões / diretrizes definidos pelo CUA.