The lesser of 64 Mbytes and 1/2 of physical memory.
1 to one-half of physical memory
Yes, unless dynamic reconfiguration operations that add or delete memory occur. At that point, the value is reset to whatever was provided by /etc/system or was calculated from the new physical memory value.
Maximum value is the lesser of 64 Mbytes and 1/2 of physical memory.
When more aggressive scanning of memory is desired during periods of memory shortfall, especially if the system is subject to periods of intense memory demand or when performing heavy file I/O.