Access key
From Guild Wars Wiki
An access key is a 25-character code that accompanies all Guild Wars products. Every game box comes with a unique access key that is used to register an account. Once an account is activated with an access key, further codes can be added to the account to unlock additional game features (such as additional campaigns, or promotional items). Once an access key is registered to an account via the game client, it is forever tied to that account and can no longer be added to any other accounts.
Access keys need not be re-registered on other computers or upon a reinstall - the keys are tied to an account and are stored on ArenaNet servers.
Purchases made in the Guild Wars In-Game Store also comes with access keys; the difference is that the access keys are automatically registered to the current account. Once a purchase is confirmed, the access key will be be added and cannot be removed or transferred.
Access keys for promotional in-game items have been made available by ArenaNet via pre-orders, magazines, and game conventions.
When dealing with support regarding account-related issues, they may often request that the access keys registered to an account be provided in order to prove ownership of that account. Thus, it's often handy to keep a record of them.
