It is documented and accessible. Not to the extent many people want, but enough to use it. If your CPU and motherboard combination supports remote management, you can usually turn it on by pressing Control+P during boot (launches configuration screen; see motherboard's manual if it doesn't) and then use freely available software like Manageability Commander from Intel's site to manage the PC.
Mandatory disclaimer: it's highly recommended to keep the network port with active management interface isolated in a separate network with no internet access.
Mandatory disclaimer: it's highly recommended to keep the network port with active management interface isolated in a separate network with no internet access.