There were many existing implementations of BASIC for CP/M, such as Digital Research's CBASIC and the third-party ZBasic, but they followed the earlier 1970s model of compilers that were fed source code prepared in a separate text editor.
Instead, Locomotive Software optimised it for business use, with, for instance, full ISAM random-access file support, making it easier to write database applications.
This patched Mallard BASIC, replacing the redundant LET keyword with LEB, which could be followed by a wide variety of parameters to allow sophisticated graphics (for the time) to be drawn on screen, saved to disc, printed, and so on.
The name was short for Roland Perry's EDitor, the program having been quickly written by Roland Perry, the Amstrad executive running the computer product development, when it was realised that CP/M-80 came with no usable full-screen editor, but users had a requirement to edit configuration files.
The same problem was apparent with DOS Plus and MS-DOS supplied with IBM-compatible Amstrad computers, but the RPED for those machines was written in Intel 8086 assembler, and not Mallard BASIC.