Compost mulch is piled and aged bark mulch. After application composted mulch loses its volume at a slower pace than fresh mulch. Compost mulch also does not promote as much competition for nitrogen between the microbes and plants. Furthermore, composted bark mulch has a tendency to be more uniform in size.
Bell, N., Sullivan, D.M. Cook, T.,(2009). Mulching Woody Ornamentals with Organic Materials. Retrieved from: https://catalog.extension.oregonstate.edu/ec1629