Content IDs are, as the name implies, used to manage content in Final Fantasy XI. There are two types of Content ID: account IDs and expansion IDs.
Account IDs are used to create characters. A monthly base fee is charged for your first character, and you may add additional characters for a smaller additional fee up to a maximum of 16.
Expansion IDs come with expansion packs; they are a one time activation without an extra monthly fee. They automatically unlock the expansion content for all characters on your account.
The fees for account content IDs for each region are:
Region Base Fee Additional Character North America $12.95 $1 United Kingdom £8.99 £0.70 France,Germany & Italy €12.95 €1 Japan ¥1280 ¥100
It is possible to manually suspend Content IDs and reactivate them later (provided certain conditions are met). During suspension you cannot access your characters and the monthly fee will not be charged. Content IDs may also be temporarily or indefinitely suspended by Square Enix in response to violations of the Terms of Service.