"Account linking" is the act of associating a Kintone account with a guest account when a user with a Kintone account is invited to a guest space in a different Kintone environment as a guest user.
By linking accounts, users can access guest spaces they are invited to from the main page of their own Kintone environment, even though the guest spaces are in a different Kintone environment. In addition, since the guest account linked to a user's Kintone account is not counted as a licensed guest user, the guest account for the user becomes available for free.
Actions by guest user: Linking your guest account and Kintone account

Example:
This example explains a case where you have purchased the license for five guest users, out of which three are in use.

• If no accounts are linked: the remaining number of available guest users is two.

• If one of the guest accounts is linked with a Kintone account of a different environment: the remaining number of available guest users is three.

Note that, even if a guest account has become available for free as a result of account linking, your billling amount will not change automatically.
If you want to reduce your billing amount, you need to reduce your number of guest accounts.