In enterprise environments, Windows operating systems are typically activated via volume licensing agreements. Microsoft provides two primary methods for this purpose: the Key Management Service (KMS) and the Multiple Activation Key (MAK). Both methods license Windows installations in compliance with legal requirements, but differ in how they work and where they are used.
With KMS activation, the company sets up its own KMS server on the internal network. All KMS clients on the network then automatically enable themselves via this server and renew their activation at regular intervals (every 180 days by default).
In contrast, MAK activation takes place directly via Microsoft’s activation servers on the Internet. Each MAK key has a limited activation quota.
Windows installations deployed via volume licensing media are preconfigured as KMS clients by default. They contain the corresponding generic KMS client setup key (GVLK) and generally require no further configuration. The system automatically attempts to find a KMS server on the network and enable itself there.
However, if a system is converted from another licensing model (e.g., MAK, retail, or KMS host) to KMS client mode, the appropriate GVLK must be installed manually. This key instructs Windows to use the internal KMS service for activation going forward. Microsoft publicly documents the GVLKs and makes them available for every supported Windows edition.

Microsoft provides a full overview.

By default, ACMP includes Microsoft's standard KMS product keys, which are listed in the workspace under OS Deployment > Windows Deployment > Product Keys.

Add product key

If you want to add a purchased product key—whether it is a MAK or KMS key—navigate to Product Key > Add in the ribbon bar.

A wizard will open where you can enter all the info about the product key.

On the next page, enter the product key, quantity, architecture, operating system, and edition.

Under Used for, you can then specify, depending on the product key, the type of licensing for which the key is being used:

• Installation
• Activation

• Installation and activation

Then click Done.