REXX itself was a language made by IBM used for all manner of purposes, AREXX was a slightly simplified take on that for the Amiga. A number of programs on the Amiga would have an AREXX "port" which provided a set of APIs which could be called from AREXX to control aspects of the application. This meant that AREXX was often used to control multiple applications or chain them together (a bit like a batch control language used to achieve an objective by having each of the programs perform part of some greater whole).