In BSD, these parameters are generally objects in a management information base (MIB) that describe tunable limits such as the size of a shared memory segment, the number of threads the operating system will use as an NFS client, or the maximum number of processes on the system; or describe, enable or disable behaviors such as IP forwarding, security restrictions on the superuser (the "securelevel"), or debugging output.

[1] In OpenBSD and DragonFly BSD, sysctl is also used as the transport layer for the hw.sensors framework for hardware monitoring,[2] whereas NetBSD uses the ioctl system call for its sysmon envsys counterpart.

Historically, although kernel variables themselves could be modified through sysctl, the elements comprising the MIB of the sysctl tree were hardcoded at compile time, and as of 2019, it's mostly still the case in OpenBSD (with some exceptions like hw.sensors, which manages and provides its own dynamic subtree).

The sysctl system call does exist on Linux, but it has been deprecated and does not have a wrapper function in glibc; it is usually unavailable due to many distributions configuring the kernel without CONFIG_SYSCTL_SYSCALL; so it is not recommended for use.

In FreeBSD, NetBSD, OpenBSD, DragonFly BSD, and Darwin/Mac OS X, the parameter net.inet.ip.forwarding can be set to 1 to enable this behavior.