It covers the Software or Hardware you have bought/Purchased.
In the case of free OS update and Upgrades "purchased" covers the free items as well.
If the item if for a Mac then it covers the amount of Mac that you are allowed to have that piece of Software installed on.
So for instance an OS update that includes an Update to the Mac version of Messages only covers that machine.
If that Update is applied to another Mac then that machine is covered.
However any version on a iOS device like an iPad of iPhone is not covered by the MacOS update's Apple Care.
8:29 pm Monday; November 13, 2017
G4/1GhzDual MDD (Leopard 10.5.8)
MacBookPro 2Gb (Snow Leopard 10.6.8)
Mac OS X (10.6.8),
iPhone 6 iOS 11.x and an iPad (2)