For those unfamiliar with the device that comment might be too cryptic (it was for me), but meanwhile I found its FORTH manual [1], where it says:

"Products designed by Information Appliance Inc. (IAI), such as the Canon Cat, have a number of unique features. One of them that directly affects third-party software development is the principle of editor-based software."

[1] http://www.canoncat.net/cat/Cat%20tForth%20Documentation.pdf