From abcm2ps web: "abcm2ps is a command line program which converts ABC to music sheet in PostScript or SVG format. It is an extension of abc2ps which may handle many voices per staff" (http://moinejf.free.fr/)

This can also be done using abcMIDI yaps utility (T2736), so why would be want two packages doing the same thing?

The mantainer of abcMIDI has the answer: "Since abc2mps is becoming the main standard for creating PostScript files from ABC files, less effort will be devoted to supporting the yaps software. (I will fix problems where yaps crashes with a segmentation error, but I will not try to bring it to the same level as abcm2ps)" ( http://abc.sourceforge.net/abcMIDI/)

So, for converting abc music files into SVG or PostScript format, abcm2ps provides the best tool, whith more features, like support for many voices per staff, while in order to convert abc to midi files or just formating abc files, abcMIDI is the way to go - abcm2ps couldn't do that-, using the provided abc2midi and abc2abc command line utilities.