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 the value provided by /etc/system or calculated from the new physical memory value.
The maximum value is the lesser of 64 Mbytes and 1/2 of physical memory.
When more aggressive scanning of memory is preferred during periods of memory shortfall, especially when the system is subject to periods of intense memory demand or when performing heavy file I/O.