In the SunOS release 5.7 software, SWAPFS is the default swap device when the system boots or you create additional swap space. This swap device uses physical memory as swap space, but also requires physical swap space on a disk.
In SunOS release 4 systems, the default physical swap device depends on the system configuration. Standalone systems default to sd0b and diskless systems get their swap files from the bootparam server. The SunOS release 5.7 software uses the swap file as the default dump device instead of specifying a file on disk.