Volume licensing
How organisations buy licences in bulk, and how the vendor tries to keep track of them without every PC phoning home.
Microsoft is the textbook case, with schemes like Open License, Select and the Enterprise Agreement. Windows XP brought product activation to home users in 2001, but the corporate edition got a volume licence key that skipped activation entirely. One such key, starting FCKGW, leaked before XP even reached the shops and became the most widely pirated key in Windows history.
Vista closed the gap in 2007 with two mechanisms. A MAK activates once against Microsoft and counts down with each machine. KMS is a local server on the corporate network that clients must check in with regularly, or their activation lapses after 180 days. KMS also refuses to activate anything until it sees a minimum number of clients, so nobody runs one at home for a single PC.