The Clusterboard is a PINE64 board that makes it possible to set up a compact cluster of headless ARM-based single-board computers. It can hold up to a total of seven SOPINE or SOEDGE modules. The Clusterboard is an open-hardware project, but please note that it does not mean this project is "OSH" compliant.
Although there is a separate wiki page that describes SOPINE modules, this page will describe both the Clusterboard and, to a certain extent, the SOPINE modules. There is a user guide for the Clusterboard in this forum thread.