Starting with Mac OS X Server 10.3, config files control certain sysctl/kernel parameters, such as the maximum number of processes, maximum number of open files, and so forth. Apple installs one config file to specify settings that differ between Mac OS X and Mac OS X Server:
/etc/sysctl-macosxserver.conf
If you want to override any of these settings, you should read the instructions contained in that file, but do not modify that actual file. Instead, put the settings you want to change in a new file at this pathname:
/etc/sysctl.conf
The new file bearing this name will be processed after the Apple-provided one, and your settings will thus take precedence. For more details about what settings are available, see the man page for sysctl(8).