Microsoft has developed different types of keys so it suits a different kind of requirements of companies running Windows.
- MAK: Multiple Activation Keys
MAK keys are used to activate a specific number of devices. The count is pre-configured as a deal between Microsoft and the Enterprise. Every time a device is activated using the MAK key, this is what happens:
- The connection is established to Microsoft's own activation service;
- The key is then verified, i.e. validated if any more copies can be activated using that key;
- If passed, then 1 is subtracted from the number of activations still available.
However, there is a huge drawback with MAK keys. If the device goes through re-installation or the hard drive is wiped clean, the count is not returned or increased. This is specifically useful for clients where re-installation is rare, and the machine stays as is for a very long time. (I'd highly recommend using MAK keys for Virtual Machines)
- KMS: Key Management Services Keys
If a company wants to keep a tap on their Windows device and make sure these computers get back to the corporate network from time to time and do not need to connect to the internet, KMS is the way to go. Instead of going through Microsoft Activation Server, KMS keys go through corporate KMS servers.
Enterprise is given KMS key using which they need to configure an in-house server using Microsoft's Software Licensing Service. So this can be the only device which stays connected with the internet. In short, the KMS server grabs the license from the client devices and then verifies it with the Microsoft licensing service.
The advantage you have with the KMS activations is in when Windows is installed on that computer again, the keys can be used to reactivate the same or another computer. This is why the devices that activated using KMS keys need to connect every 180 days, otherwise, they would expire.
KMS keys are used on devices which usually do not leave from the corporate network, or at least not for a very long period.